The hypothesis that brook sticklebacksCulaea inconstansand fathead minnows Pimephales promelas are competitors was tested in field enclosures containing young-of-the-year individuals from both species. Growth was monitored for 6 weeks, then final weights and gut contents were examined. Fathead minnows were relatively unaffected by brook sticklebacks. Neither dietary composition nor growth was affected in enclosures containing only fathead minnows or a combination of fathead minnows and brook sticklebacks at similar density. The presence of fathead minnows did, however, stimulate growth of brook sticklebacks; this was possibly related to the relatively higher mortality of fathead minnows within the jointly occupied enclosures and the resulting reduced absolute density of fish. The diet of brook sticklebacks also became more diverse in the presence of fathead minnows. In enclosures containing only sticklebacks, the fish consumed primarily copepods. These data demonstrate that growth of brook sticklebacks may be sensitive to both intra- and interspecific competition. This apparent sensitivity of growth to the presence of fathead minnows may account for the observed spatial distribution and migratory habits of brook sticklebacks.