To determine how moonlight and daylight affect hydroacoustic estimates of fish abundance, we used a dual-beam transducer and echo integration to survey pelagic fish (primarily Bonneville ciscoesProsopium gemmifer) in Bear Lake, Utah and Idaho. During the new moon, the fish were dispersed (not schooling) below the thermocline, chiefly at the depths of 10–20 m. At full moon, they were dispersed but much closer to the bottom, where they were difficult to detect. Acoustic estimates offish density and biomass during full moons were approximately 50% of values derived during new moons. A diel survey during a new moon indicated that fish were widely dispersed in the water column at night, but formed schools at dawn. Our study indicated that light conditions must be standardized to insure consistent and comparable population estimates of some pelagic fishes.