Fiserv Inc. says its MyCheckFree.com payment site has become a key part of small billers' bill-payment strategies.

The Web site handles payments on behalf of billers, and though it was originally designed to appeal to any kind of company, online bill payment is now a common element of larger billers' Web sites.

As a result, big billers have moved off the Fiserv site, but smaller ones have moved in, said Lori Stepp, a director of product management and marketing for the electronic biller services division of Fiserv's CheckFree business.

In the early part of the decade, when online bill pay was a much newer concept, "we built that MyCheckFree site because it was an easy plug-and-play to get billers up and running quickly and inexpensively," Ms. Stepp said in an interview last week. Today, "there are still some smaller billers who may not be willing to invest in a fully customized site, and referring consumers to MyCheckFree meets their needs."

Fiserv acquired the Atlanta bill-pay technology and service provider CheckFree Corp. in December of 2007.

MyCheckFree.com has 5 million active users. It uses the same bill-pay software Fiserv offers directly to banks; the only feature the site lacks that the company offers to banks is the ability to send check payments to billers not connected to its CheckFree unit electronically.

The number of users increased 11% last year and 10% in 2007, Ms. Stepp said.

Though people can pay multiple billers, just as they can with the bill-pay services offered through banks, most of MyCheckFree.com's users pay just a single bill every month, to the biller that referred them to the site, Ms. Stepp said.

Jennifer Roth, a research director with the global payments practice at TowerGroup Inc., a Needham, Mass., independent research firm owned by MasterCard Inc., said Fiserv and other providers of bill-pay services have done a good job of convincing big billers to add payment capabilities to their Web sites.

Most companies that issue more than a million bills a month have a system in place, as do many midsize billers, she said.

But most billers issue fewer than 50,000 bills a month, Ms. Roth said, and for these companies the business case for adding bill-pay features to their Web sites may not be as compelling. Online bill payment "has to fit into their budget … there has to be a definite ROI around it," she said.

One reason is that online bill payment has lost some of its speed advantage over receiving payments by check, now that many checks are converted to images and routed across image exchange networks.

"Check clearing is faster now; Check 21 also makes things move fast," Ms. Roth said.

A big part of the ROI equation for small billers is whether enough people would use the service to justify the cost and effort of adding it to a biller's Web site, instead of simply directing people to MyCheckFree.com.

Utilities are a good example of billers that may not need their own bill-pay service, Ms. Roth said.

"Utilities are basically regional, or even local in some cases, so their constituents and their customer bases are fairly small," she said. "So for a user who needs to pay Utility X in the far hills of North Dakota somewhere, … [MyCheckFree.com] may be the only option that they're ever going to have. I could see where it would continue to grow as they add on more and more billers, especially on the smaller side."

MyCheckFree.com is filling an important niche, and its growing user base speaks to the increasing popularity of paying bills online, Ms. Roth said.

"Five million active users that do at least one payment a month is a pretty good marketplace," she said.