We tested the response of kokanee Oncorhynchus nerka to strobe lights. Testing was conducted on wild, free-ranging fish in their natural environment (i.e., the pelagic region of two large Idaho lakes). Split-beam hydroacoustics were used to record the distance kokanee moved away from the lights as well as the density of kokanee in the area near the lights. In control tests, where the strobe lights were lowered into the lake but kept turned off, kokanee remained within a few meters of the lights. Once the lights began flashing, the distance between kokanee and the light source increased and was highly significant when compared with the control transacts (P < 0.001). Kokanee moved an average of 30-136 m away from the lights in waters with Secchi transparencies from 2.8 to 17.5 m. Kokanee densities near the lights were significantly lower (P < 0.001) when the lights were turned on than in control samples with no lights flashing and densities dropped between 72% and 100%. Flash rates of 300, 360, and 450 flashes/min elicited strong avoidance responses from the fish. Kokanee remained at least 24 m away from the lights during our longest test of 5 h 50 min. Kokanee appeared to be responding to flashes that were less than 0.00016 lux (about 2.96 x 10 super(-6) micromoles/m super(2)/s) above background lighting.