Little is known about what strategies are cost-effective in increasing participation among physicians in surveys that are conducted exclusively via the web. To assess the effects of incentives and prenotification on response rates and costs, general internists (N = 3,550) were randomly selected from the American Medical Association (AMA) Masterfile and assigned to experimental groups that varied in the amount of a promised incentive (none, entry into a $200 lottery, $50, or $100) and prenotification (none, prenotification letter only, or prenotification letter containing a $2 preincentive). Results indicated that the response rates were highest in the groups promised $100 and $50, respectively. While the postal prenotification letter increased response rates, the inclusion of a small token $2 preincentive had no effect on participation. Further, unlike mail surveys of physicians, the $2 preincentive was not cost-effective. Among physicians, larger promised incentives of $50 or $100 are more effective than a nominal preincentive in increasing participation in a web-only survey. Consistent with prior research, there was little evidence of nonresponse bias among the experimental groups.