Second and fourth instars of Culex quinquefasciatus Say and Aedes aegypti (L.) and fourth instars of Culiseta morsitans (Theobald) were fed five size classes of latex particles ranging from 0.56 to 5.75 microns. Each experimental run was done on two size classes of particles mixed in different proportions of 10:90, 50:50, and 90:10. Both the gut contents and the food mixtures were analyzed by flow cytometry, and the particle percentage composition was determined. The null hypothesis that no particle selection existed among instars or species was rejected for all species. Small instars ingested disproportionally more large particles in all suspensions. Cs. morsitans and Cx. quinquefasciatus showed variability in their responses. Ae. aegypti almost always ingested more of the larger particles. Differences in particle size selection were related to head morphology and feeding mode. Under experimental conditions, two parameters influenced particle size selection: the relative amount of small particles offered and the size difference between small and large particles. Flow cytometry analyses are recommended for quantitative studies of field material.