Plastic contamination is an increasing environmental problem spreading globally to even the most remote aquatic and terrestrial habitats and in particular, microplastics (< 5mm) represent an uncertain threat for organisms or for the ecological balance. In this thesis, I firstly assessed the microplastic pollution in a stretch of the River Inn close to Innsbruck and secondly, tested the microplastic toxicity in the absence or presence of pesticides. To reach these objectives, shore sediments of the river were analysed for the occurrence of plastic debris and toxicity tests were done with Daphnia magna and different glyphosate pesticide solutions (glyphosate acid, glyphosate-IPA salt, Roundup® herbicide), combined with microplastics (PE spheres, PET filaments). As expected, plastic pollution was found to be prevalent in the River Inn (0.33 items g-1 dry weight), with microplastics recovered and identified in almost every sediment sample analysed. In agreement with the global production rate and with previous studies on the abundance of microplastics in the environment, the major plastic types identified by Raman-Spectroscopy were polystyrene, polypropylene and polyethylene. Fibers of different colours represented the most common found plastic-like debris in this study (60%), but unfortunately were also the least Raman identifiable (8%). The toxicity tests showed that PET fibers at the concentrations used (10 fibers mL-1, 30 fibers mL- 1) caused lower mortality of daphnids than PE microspheres (1.1 x 106 spheres mL-1). Though Roundup was expected to cause the highest mortality rate, this was not the case. Generally, it was not possible to identify any clear synergistic or antagonistic toxicity-trend in combined treatments of pesticides and plastics. The variability found among the performed toxicity tests could be linked to discrepancies in the solubility of glyphosate based chemicals, differences in the chemical additives (non specified “inert ingredients”) of the applied glyphosate formulations, as well as to clone-dependent sensitivity of daphnids. The outcomes of this study underline the large variety of physiological and chemical interactions and processes coupled to microplastics and their ubiquitous occurrence in the environment. They also stress the need for further research to unravel this complex processes and to establish science-based environmental risk assesments to counteract the emergence of a plastic world.