The effect of different diet on Oppia denticulata was studied during a 4-month laboratory experiment, based on daily observations. This species is known as microphytophagous (i.e. feeding on bacteria and/or fungal hyphae and/or algae), like other species of Oppiidae, but our observations show its wider diet. We tested 11 types of food, including homogenous diet [green algae, lichens, fungi (dry yeasts, Trichoderma viride, Penicillium chrysogenum), nematodes], and combined diet [green algae and nematodes, lichens and nematodes, dry yeasts and nematodes, T. viride and nematodes, P. chrysogenum and nematodes]. The basic population parameters (fertility, mortality, sex ratio), time of development, population growth in four months and body length were compared between all groups. Observations on biology of O. denticulata were also made. The most favourable diet for O. denticulata was a combination of T. viride and nematodes, which allowed the highest population growth (about 50 times higher than the least favourable food, a combination of P. chrysogenum and nematodes), due to highest fertility and shortest development. Good population parameters and largest size were obtained on yeasts (served alone or with nematodes). This species did not do well on P. chrysogenum, probably because it produces penicillin.