We investigated the effects of prey size and type (sedentary vs mobile) on prey preference in a predaceous stonefly, based on choice experiments and observations of the predator-prey interaction. We presented three size classes of black fly larvae (Prosimulium) to various sizes of the perlid stonefly Hesperoperla pacifica in laboratory experiments. Analysis by preference indices (a, e) indicated that small predators preferred small prey, large predators preferred large prey, and H. pacifica of intermediate size exhibited a weak, non-significant trend toward preferring prey of intermediate size. These results using sedentary prey, black fly larvae, are compared to previous results using a mobile prey, the mayfly Baetis. Encounter, attack, capture rates and handling times differ as a function of predator size, prey size, and prey type. The results of preference trials generally can be predicted from differential encounter, attack and capture rates, and similar preference curves may have different underlying causes. Small H. pacifica preferred small prey, mainly due to a much higher capture success. Large H. pacifica preferred large prey, due principally to higher encounter and attack rates. Offered a choice between Baetis and Prosimulium, stoneflies exhibited lower encounter and attack rates with Prosimulium, but much higher capture success, resulting in a net preference for black fly larvae. We suggest that laboratory results for attacks, captures and handling time can reasonably be extrapolated to animals in nature, thus lending insight into preference estimates based on gut analysis. However, encounter rates inevitably are influenced by details of the laboratory design and may bear little relation to nature.