The large quantities of plastic particles reported from lakes and rivers are the cause of an increasing number of microplastics studies in freshwater environment. Microplastics ingestion by freshwater biota has been demonstrated but the potential harm caused by microplastics is not clear yet. Rotifers are important primary consumers in freshwater environment, linking the primary producers and higher trophic consumers. Filter – feeding organisms such as rotifers are unselective and thus highly vulnerable to microplastics exposure. In this study we evaluate the effect of different sizes and types of microplastics (1-, 3- and 6- µm PS microspheres and 5-25 µm PA fragments) on two life history traits : population growth rate and reproduction, of two cosmopolitan rotifers: Brachionus calyciflorus and Brachionus fernandoi. The different treatments were combined with high and low concentrations of algae as food, in order to simulate optimal and stressed natural conditions. A decrease in intrinsic growth rate and reproduction was found in B. calyciflorus exposed to 3- µm PS microspheres in combination with high and low concentration of food algae. Contrastingly, B. fernandoi showed decreasing growth rate just when exposed to low concentration of food in combination with 3- µm PS microspheres. Smaller and larger PS microspheres (1-, 6-, µm) had weaker but not significant effects and the PA fragments had no effects on intrinsic growth rate and reproduction of both rotifers species. Additionally, we tested different concentrations of 3- µm PS microspheres in combination with varied food algae species. The ratio between food algae and microplastics, as well as the size and nutritional quality of food algae, may have a key role to understand the indirect effects of microplastics on rotifers.