The effects of various factors on the efficiency of the Gee minnow trap to sample populations of mosquitofish, Gambusia affinis, in both rice field enclosures and a laboratory tank were assessed. Immature green sunfish, Lepomis cyanellus, were assessed in the laboratory tank only. Trap efficiency (percent catch) was greater for larger mosquitofish [greater than or equal to 35 mm total length (TL)] than for smaller mosquitofish. Immature green sunfish (25-40 mm TL) were caught at higher rates than either size class of mosquitofish. In the laboratory, trap efficiency increased with increasing mosquitofish density, but density had no effect in field enclosures. Vegetation did not affect trap efficiency. Minnow traps, lying on the substrate, caught a similar percent of available mosquitofish when water depths were 8 and 16 cm but a significantly smaller percent when 24 cm. In contrast, green sunfish, assessed at 8 and 24 cm, were trapped at a higher rate at 24 cm. These results demonstrate the importance of determining the effects of environmental factors on trap efficiency before using direct trap counts to assess effects of these factors on fish abundance.