Investigation of predator-prey relationships that involve aquatic insects has received little attention, and aspects including feeding rates and prey selection of many predators are poorly understood. Adult giant water-bugs, Lethocerus americanus, and larval giant water scavenger beetles, Hydrophilis triangularis, are large predatory aquatic insects that typically inhabit pools or ponds with little to no flow. Both insect predators are known to prey on a variety of aquatic organisms including invertebrates and vertebrates. Laboratory trials were conducted to determine feeding rates and prey selection of fish by adult giant water-bugs and larval giant water scavenger beetles. Six western mosquitofish, Gambusia affinis, of two size classes, juvenile (<20 mm) and adult (>20 mm), were placed in aquaria with a single predator. Once per 24 hour period, predator survival was recorded along with the quantity and size of fish consumed. Each of the insect predators consumed a significantly greater number of mosquitofish from the adult size class with giant water-bugs consuming 0.13 fish per day and water scavenger beetles consuming 0.46 fish per day. Insect predation of small fish species could be substantial and because size selective pressures exist, population structure of fish populations may be affected.