The gut content of nearly 2000 specimens of Enoplus brevis was quantitatively analysed. E. brevis is an opportunistic omnivore. Main food categories found were cyanobacteria, diatoms, oligochaetes, nematodes, and rotifers. With a few exceptions, the diet of females and males was identical. Predominantly, juveniles fed on cyanobacteria and adults on animal prey. Experiments revealed that living oligochaetes are attacked and preyed upon. The quantities of the food categories eaten by Enoplus varied according to their ambient abundance, both temporarily and spatially on a small scale. At least the nematode prey was strongly selected: some species were significantly preferred and others avoided.