The problem began at 7:30 a.m. PDT, according to Wells Fargo spokeswoman Andrea Mahoney. The company expects to fix the problem by the end of the day, she said.
"We are currently experiencing network connectivity issues," Mahoney said. "Some customers are experiencing intermittent access to account information."
The outage is the second major one for Wells Fargo this year. Some customers hadlogging into their online accounts for several days in April.
Wells Fargo, which is the fifth-largest bank in the United States, said problems with its mainframe computers had caused the April glitch. Mahoney would not discuss the cause of Friday's problem but called it an "isolated incident."
Of Wells Fargo's more than 20 million customers, 3.2 million use the bank's online services, according to Mahoney.
Online banking is one of the most popular consumer uses of the Web. Nearly 20 percent of all U.S. households, or 22 million, use the Web to do things like check account balances, transfer money between accounts, and pay bills, according to Online Banking Report, an industry newsletter.