The effect of salinity levels, water source and prey density on the predatory efficiency of mosquitofish, Gambusia affinis (Baird and Girard) against the 4^th instar larvae of culex pipiens was studied. The salinity levels of 5, 10 and 15% significantly affected the rate of predation of G. affinis. All fishes have died at a salinity level of 15%. However, the predatory efficiency of both males and females was significantly higher at a 5% salinity level than that at 10%, despite fish sex or size.  The number of mosquito larvae consumed by a female was higher than male at a 5% salinity level, but the reverse was observed at a salinity level of 10% despite the fish size. The type of water in which mosquito larvae or fish were bred affected the predatory efficiency of G. affinis for both male and female fishes and for all sizes of fish. Distilled water was the highest significant favorable water for predation. The density of mosquito larvae affected the predatory efficiency of G. affinis. Results showed that mosquito larvae at a density level of 40 larvae/200 ml water were the most favorable density for predation by G. affinis despite sex or size of the fish.  In general, females were more efficient in predation than males despite fish size or prey density.