SAN FRANCISCO Companies across the U.S., worried that cyberspace will be terrorism's next battleground, have shored up security since Sept. 11.

About 77% of businesses improved defenses against hackers, viruses and other attacks, says a survey of 233 corporations by Computer Economics.

Such threats are real. Cyberspace attacks jumped 64% from a year ago, says security firm Riptech especially from countries such as Iran and Pakistan that are known to harbor terrorists.

Also, 90% of big corporations and government agencies responding to a survey by the Computer Security Institute and FBI said they uncovered computer security breaches in the past year.

Earlier this month, the FBI warned America Online, Microsoft and other Internet service providers about possibly imminent hacker attacks. AOL and Microsoft took steps to shield their 43.7 million subscribers. No damage was reported. Experts expect more such warnings as Sept. 11's first anniversary nears.

The cyberspace threat is greatest for the nation's 5.6 million small companies, which employ half of all workers and are the economy's backbone.

Small firms often lack money to hire full-time information-technology professionals and rarely think they are likely terrorist targets.

Still, Computer Economics says companies with less than $1 million in annual revenue were the biggest proportion of those that bolstered security with:

Anti-virus programs. In Ann Arbor, Mich., chiropractor Darren Schmidt used software before Sept. 11 to hunt for viruses contained in e-mail. But after the attacks, Schmidt learned the program wasn't getting updated often enough to guard against newly hatched viruses. Around the time of the attacks, Schmidt had one virus attack that shut down his computer for a week. Now Schmidt, who keeps contact information for 200 patients on his computer, updates his software daily.

File-backup gear. In Charlotte, outplacement firm Forum Group was sporadically copying computer files before the attacks. "We thought we were in pretty good shape," says co-owner Bill Crigger. But employees fretted about security after Sept. 11, so Crigger hired a consultant who recommended a daily backup schedule. Companies in remote places often think they don't need to worry about terrorists targeting their computer networks. "Everyone believes it won't happen here," says Jerry Rackley, a publicist in Stillwater, Okla. Yet Oklahoma residents felt immune to terrorism until the 1995 bombing of the Alfred P. Murrah Federal Building in Oklahoma City.

"The reality is, you have to prepare for the worst and hope for the best," Rackley says.