Four protistan species, Tetrahymena pyriformis, Chilomonas paramecium, Euglena gracilis, and Chlamydomonas reinhardti, were fed individually or in pairs to the ciliate Stentor coeruleus. Making use of a new definition of food preference which does not confound catchability with choice, this species' food preferences were measured by comparing the mean consumption of a population of stentors when each prey species was present alone with the mean consumption when a pair of prey species was present. S. coeruleus was found to exhibit consistent food preferences, preferring protozoan to algal prey while indicating no preference when choosing between algal or between protozoan prey. Preferences were not correlated with prey size.