Laboratory exposure studies allow to investigate the impact of microplastics on marine biota, but commonly lack a procedural control, i.e. assessing the effects of natural microparticles. In two experiments with the mussel Semimytilus algosus, we compared the effects of clay vs. polyvinyl chloride (PVC) and celite vs. polymethylmethacrylate (PMMA), respectively, at concentrations of 1.5, 15 and 150 mg l(-1) After more than 60 days, no effects on respiration and clearance rates, mortality and byssus strength were observed. However, in mussels exposed to PVC the Body Condition Index was 34% lower at 150 mg l(-1) than at 1.5 mg l(-1). Furthermore, at 15 mg l(-1), mussels exposed to microplastics produced over 40% less byssus than those exposed to natural micro particles. This suggests that mussels react differently to natural microparticles and to microplastics, but only at high particle loads that exceed current environmental microplastic concentrations by orders of magnitude.