The effect of the presence of scuba divers conducting visual point count surveys on the abundance and size distribution of fishes estimated with dual-beam hydroacoustics was examined at a petroleum platform in the northern Gulf of Mexico during November 1991 and February, March, and June 1992. Mean density of fishes declined 41–77% for all sampling periods when scuba divers were present. Target strengths also dropped significantly for all sampling periods when scuba divers were present. Our results show that fishes at petroleum platforms avoid scuba divers. Therefore, visual surveys of fish at petroleum platforms and other similar environments should be supplemented with hydroacoustic assessments.