The online behavior of a small but growing number of computer users in the United States is monitored by their Internet service providers, who have access to every click and keystroke that comes down the line.

The companies harvest the stream of data for clues to a person's interests, making money from advertisers who use the information to target their online pitches.

The practice represents a significant expansion in the ability to track a household's Web use because it taps into Internet connections, and critics liken it to a phone company listening in on conversations, but the companies involved say customers' privacy is protected because no personally identifying details are released.

The extent of the practice is difficult to gauge because some service providers involved have declined to discuss their practices. Many Web surfers, moreover, probably have little idea they are being monitored.