Exceptional spring spawning of yellow perch (Perca flavescens) in an inland coastal lake resulted in very high larval fish densities (up to 41 900/1000 m3) and facultative predation by adult female Diacyclops thomasi on newly hatched larvae (maximum predation density: 8740/1000 m3), as evidenced by cyclopoid attachment to fish larvae in field collections. This predation suggests that larval fish mortality and possibly cyclopoid population growth are affected. Opportunistic cyclopoid predation on fish larvae may also alter zooplankton community structure by changing the feeding pattern of cyclopoids. Lack of significant predation on other species of fish larvae is probably due to temporal and spatial asynchrony of predator and prey, low probability of encounter, robust nature of certain fish species, and effective swimming and escape ability.