Microplastics (MP) are plastic particles ≤ 5 mm, which have become prevalent in the environment and have negative effects on it. They enter the environment either directly through discharges of sewage plants or resulting from mechanical transformation of large plastic parts. Due to their small size, they pose a risk to have negative effects on organisms. To date, there are still many unknowns regarding the potential bioavailability for small aquatic invertebrates. Since most of MP are lighter than water, we do not know whether it is accessible to all organisms and how the size of the MP affect the bioavailability. The aim of this study was to determine bioavailability and effects of the MP in size range from 20 – 1000 μm on model organisms Daphnia magna and Artemia franciscana. Selected model organisms were exposed for up to 48 hours or two weeks at concentration, 100 mg/L. Model organisms ingested all types of tested MP. However, MP does not affect the mortality. Our results show that bioavailability is related to the particle size: smaller particles were ingested more readily than larger. They also have a potential impact on the size of the organisms.