The authors examine prominent placement of search engines' own services and effects on users' choice of destinations. Using a natural experiment in which different results were shown to users who performed similar searches, they find that Google's prominent placement of its Flight Search service increased the clicks on paid advertising listings by more than half while decreasing the clicks on organic search listings by about the same quantity. User substitution disproportionately affected the most visited travel sites, reducing use of organic listings sending no-charge traffic to those sites by lowering their prominence and perceived importance, while highlighting paid listings to the same sites. The authors consider the implications of such changes for online marketers and for search engine operators.