ABSTRACT: Plastic pollution is present in aquatic systems worldwide. While numerous studies have investigated microplastic interactions with marine organisms, microplastic effects on freshwater organisms, especially insects, have been rarely studied. Previous studies have mainly focused on dietary uptake of microplastics, but the presence of microplastics in animal constructions is largely unknown. To date, microplastics have only been observed in the tubes of a marine polychaete species. In freshwater systems, common caddisfly (Trichoptera) larvae build cases by using larval silk and mineral grains from benthic sediments, which are known microplastic sinks. Therefore, we examined caddisfly cases for microplastic presence. We collected caddisfly Lepidostoma basale cases in the field, disintegrated them using hydrogen peroxide, and determined microplastic polymer type through micro-Fourier-transform infrared spectroscopy. We found primary and secondary microplastics of different shapes, colors, sizes and chemical compositions (e.g. polypropylene, polyethylene, polyvinyl chloride). Thus, this is the first study to show that microplastics are present in the biological construction of a freshwater organism. Larval stages are usually more vulnerable than adult individuals, and microplastics can transport persistent organic pollutants and emit toxic leachates. In the caddisfly larval case, those substances are in close proximity to the sensitive larval body, which may be harmful for the larva and may eventually impede its development. We discuss the potential of caddisfly larval cases to act as microplastic bioindicators in freshwater habitats.