Plastic is an integral part of modern lifestyle. However, due to versatile use and poor recycling management, plastic litter is meanwhile spoiling almost all habitats on earth. Larger plastic pieces can harm animals by entanglement, suffocation, stranding, or they simply provoke injuries when animals try to escape from getting trapped. Gradual degradation of bigger plastic pieces generates micro-sized particles which can become vulnerable to small organisms. Recent studies indicated that ingestion of microplastics can impair the nutritive state of marine invertebrates. In a laboratory study we tested whether microplastics affect food quality and, in turn, nutrition in peracarid crustaceans. Marine isopods, Idotea emarginata, were fed for eight days with natural food, the brown alga Fucus vesiculosus, and an artificial diet which contained freeze dried algae embedded in agarose. Additionally, both diets were blended with microplastic particles at a concentration of 40 items per mg of food. The isopods accepted both types of food but consumed significantly (3.6-fold) more of the agar based artificial food. Addition of microplastics did not significantly alter feeding rates on both diets. Enzyme ac tivities showed high scatter and inconsistent results. Esterase (C4) and lipase (C18) activities decreases in the gut and in the midgut gland when microplastics were added to fresh algae. Activities of phosphoesterase exo-and endopeptidase showed no distinct changes in activity. Idotea emarginata compensated for impaired food quality by increasing the ingestion rates by exactly that factor by which the amount of digestible organic matter was reduced. The amount of microplastics was apparently too low to provoke such kind compensation. However, our results indicate that microplastic may differently affect digestive enzyme activities in the gut and the midgut gland of marine isopods. Further research is needed to verify whether the observed alterations affect food digestion and nutrient assimilation.