Food size selection of the mosquitofish , Gambusia affinis affinis. was measured in aquaria using juvenile stages of the mosquito, Clllex tarsalb;, as prey. Fish size varied from recently born fry to large adult females. Food size selection was positively correlated with fish size. Mosquitofish fry (6-8 111m standard length) attacked and ate primarily first and second instar larvae. Fry attacked larger instars, but attack success on these was low (0 - 50%). Fish larger than 20 mm attached primarily pupae and third and fourth instar larva. No first instar mosquitoes were eaten. Attack success for these fish was above 65'Yr) for all instars.