Changes associated with increasing body size, such as higher swimming speed and mouth gape size, visual acuity, and reaction distance to prey, may increase the range of prey available to a predator as it grows (Werner, 1974; Breck & Gitter, 1983; Dunbrack & Dill, 1983; Keast, 1985). Kennard (1995) demonstrated that small microphagic planktivore/ insectivore species were limited in prey choice by constraints in body and mouth sizes. On the whole, the fact that Ambassis agassizii fed mostly on small microcrustaceans is an indication that this species is restricted to this component of the zooplankton by mouth size and locomotory ability. Additionally, even though diets were found to differ between sampling occasions, the main dietary items were similar with only the overall proportional contribution of dietary components changing over time. It is therefore possible that resource availability had a strong influence on the diet composition of different size classes of this species. Winemiller (1989)