By converting yet another rival to a partner last week, Allpoint Network solidified its position as the dominant network of surcharge-free automated teller machines.

Rather than relying on a shared model - signing up small banks with a handful of geographically concentrated machines - the Houston unit of Cardtronics Inc. courts companies that put large volumes of ATMs at merchant sites across the country.

"The shared model has hit a ceiling," Ben Psillas, the president and founder of Allpoint, said in an interview. "What incentive do I have [as a large bank] to participate in a shared network? None. All the little guys get to feed off of you." His network charges a fixed monthly fee based on the number of participating cardholders.

The model appeals to some small banks that want to expand their ATM reach, but it offers little to large financial companies, so it is hard for networks to expand beyond the regional level.

That's what led first Fiserv Inc. and, more recently, First Data Corp. to link their surcharge-free ATM networks to Allpoint's. Fiserv's Accel/Exchange Network connects about 1,000 bank-owned ATMs through its NoSur service. First Data would not say how many banks connect their machines through StarSF, one of several services provided through its Star Network. By comparison, Allpoint has 35,000 ATMs in all 50 states.

Expanding StarSF, which operates mainly in the South and Midwest, "was obviously important to us and our customers to fill in the areas where we did not have a lot of surcharge-free play," said Kirk Ergang, the senior vice president for Star Network.

StarSF found it tough to grow, because each bank it signed up added just a few ATMs, he said. "Filling in pockets was more of a one-off exercise, and therefore we started looking for a coast-to-coast opportunity."

Mike Williams, the Fiserv senior vice president in charge of Accel/Exchange, said it encountered the same problems when trying to expand NoSur. His unit joined Allpoint in January.

Building a national, surcharge-free system is "very difficult to do" using shared, bank-owned ATMs, he said. "If you only have one or two banks that join in any given area, you really haven't accomplished anything, because there's no real value in that. Who are they sharing with? Almost nobody."

Attracting large banks, which would bring more ATMs and rapidly expand the network, is difficult, he said.

"If I have 20 machines and everybody else around me only has five, that's not an equitable balance of value there," Mr. Williams said. "The larger banks may potentially feel like 'I'm giving up too much, and I'm not getting as much in return.' That's another thing you have to battle when promoting this type of program."

Almost 200 banks with about 1,000 ATMs use NoSur, he said. The surcharge-free service has been available for a decade, but Fiserv put little effort into marketing it until April of last year, when it was formally named NoSur. At the time about 50 banks with 500 ATMs were using it.

Officials at other debit companies, including Pulse EFT Association of Houston and Shazam Inc. of Johnston, Iowa, said they are rethinking their strategies, because they have found it difficult to attract bank clients to provide additional machines.

Pulse, a unit of Discover Financial Services LLC, offers a shared ATM service called Select ATM. "Market adoption of surcharge-free ATM alliances, and networks in general, has not been widespread," Anne Rhodes, a spokeswoman for Pulse, wrote in an e-mail. "Pulse is currently considering a range of options for expanding its Select ATM program." She did not rule out joining Allpoint.

Shazam offers the Privileged Status shared ATM network. Dan Kramer, Shazam's senior vice president of marketing and merchant operations, said its network includes about 3,000 ATMs, mostly in the Midwest, and is attracting three to four new banks a month.

However, these surcharge-free networks have become less important to some bank clients in the past few years, he said, in large part because many people are using their debit cards to get cash back at the point of sale.

He also said that Shazam is "actively engaged in identifying partners" to expand its network. Its list of partners could include Allpoint, Mr. Kramer said.

Paul A. Tomasofsky, the president of Two Sparrows Consulting LLC, and a former executive at NYCE Network (now a unit of Marshall & Ilsley Corp.), said such networks enjoyed "rapid growth" in the early days "because there was a lot of appetite for some of the smaller institutions" to extend their reach. "They slowed the growth after the initial phase because they garnered the market share they were going to garner, and that was it."

Some banking companies are using a different strategy, because they have found that the issue of network affiliation only confuses customers.

Cambridge Trust Co. in Massachusetts participates in NYCE's shared ATM service, but it also offers to reimburse some customers for fees they pay to use other banks' machines.

Robert Siegrist, Cambridge Trust's marketing director, said that one of the problems with shared networks is telling customers which ATMs they can use.

"It was always difficult to communicate that. 'Look for this little symbol, and if it's there, you can use it. If not, you're going to be charged,' " he said. "Surcharge reimbursement takes that out of the equation. You can use any ATM, which makes it more convenient. People just want simplicity, and we can offer that by reimbursing."

One shared network that has fared better than the rest is Co-op Financial Services of Ontario, Calif., which was built strictly for credit unions. Started with a group of California credit unions, it now has 1,971 credit unions and about 25,000 ATMs in all 50 states. It has also augmented its shared network by adding merchant ATMs through deals with Cardtronics, TRM Corp., and 7-Eleven Inc.

With other shared programs, "there is a mixture of community banks and maybe supercommunity banks and credit unions, and there's just no affinity in those programs," said James A. Hanisch, an executive vice president of corporate development at Co-op. "It was very difficult for either the financial institution or for the network to really market significantly around that. In our case, we were pretty clear-cut in what we were trying to do, which was to build a national, surcharge-free distribution network for credit unions."