More women than men surf the Net.

That's a conclusion of online marketing firm eMarketer Inc., which issued study results Thursday saying that girls and women represented 51.6% of Americans online last year. While females make up about 51% of the U.S. population, and that ratio is expected to hold in the coming years, the proportion of girls and women on the Web should grow to 52.6% by 2008, according to eMarketer.

That's a big change from the early days of the Internet, when males dominated. As recent as 1997, boys and men made up three-quarters of Internet users.

"Cultural, societal, and Internet business trends are combining to shift the balance toward women," said eMarketer analyst Debra Aho Williamson in a statement accompanying the release of the report she authored, Women Online In The U.S.. "Because of these factors, the female majority online will become more pronounced over the next five years--and that will have a transformative effect on content, commerce and marketing."

Mary Hunt, new media director of the marketing firm Interpret-Her, says the Internet lets women get more done such as shop, plan trips, or look for homes in less time. The Net also is an alternative to the telephone, she says. "The Internet allows women to be social and pro-active at the same time," she says.

According to eMarketer, women have long embraced the Internet as a communications medium, and have shown a strong interest in online games, health content, and music. Traditionally, women have been the dominant offline shoppers, but they're shifting more of their shopping online.

The challenge for online retailers will be to develop shopping tools that mimic the offline shopping experience, Williamson said. "That means instead of offering a page of thumbnail images of blouses," she said, "retailers must use the power of the Internet to build customized shopping experiences tailored to a woman's browsing or previous buying patterns."

The gender study estimates that 52%--or 91.2 million--of the 175.4 million Americans online this year will be female. The annual growth rate of both sexes online will slow in the coming years. The female growth rate in Web use will decline from 4% last year to 2.3% in 2008. For males, the rate will decline from 3% in 2004 to 1.9% in 2008.

To reach its conclusions, eMarketer aggregates and analyzes E-business research from more than 1,700 sources and uses third-party benchmarks for demographic data such as those provided by the U.S. Census Bureau, Department of Commerce, and Internet Advertising Bureau/PricewaterhouseCoopers research, among others. The company checks its modeling and data against past models to help extrapolate growth rates.