Laboratory experiments have suggested that ingestion rates should increase with food concentration, that these functional responses should be depressed by increases in the availability of alternate foods, and that feeding preference should be shown for the most abundant of available foods. To test these hypotheses, simultaneous measures of the foraging of three species of littoral Cladocera, on two spatially differentiated food sources, were made in situ. The species investigated were Sida crystallina (O. F. Muller), Chydorus sphaericus (O. F. Muller), and Alona affinis (Leydig). The uptake rates of suspension and periphyton were measured with a double isotope technique. In situ foraging responses were determined through regression analysis. Ingestion rates were related to concentration of food, concentration of alternate food, and body size. Functional responses in situ ranged from Holling's type 2 to feeding inhibition, and the increased abundance of alternate food depressed functional responses in some cases and elevated them in other cases. Variations in feeding preference were examined in two species of cladocerans. One species (Sida) responded in a complex manner, showing increased preference for the most abundant food in one circumstance and increased preference for the least abundant food in another circumstance. The other species (Chydorus) responded to the inhibitory effect of one food source by increasing feeding preference for the alternate food. Due to the complexity of the behaviors, a multivariate in situ approach is suggested for future studies.