A laboratory experiment examined competition for bacterial food between a protist and a rotifer. The experiment utilized a randomized 2 x 2 factorial design (+/0 rotifer sp., +/0 protist sp.). When grown monospecifically, the rotifer Philodina sp, and the protist Paramecium multirnicronucleatum were similar in terms of mean efficiency of bacterial grazing in batch culture. When cultured together, the identity of the dominant species varied: Philodina dominated 3 of 5 replicates, while Paramecium dominated the remaining 2 replicates. The variable competitive outcome is predicted by variation in mean grazing efficiency when each species is raised alone.