Plastic litter is now pervasive in the aquatic environment. Several marine and terrestrial organisms can fragment plastic with their feeding appendages, facilitating its breakdown and generating microplastics. However, similar studies with freshwater organisms are extremely limited. Here, we explored the interactions between the caddisfly larvae Agrypnia sp. and polylactic acid (PLA) film. The use of plastic by larvae to build their protective cases was investigated, along with their ability to fragment the plastic film, as they do with leaf litter. Caddisfly consistently incorporated PLA into their cases alongside leaf material, they also used their feeding appendages to rapidly fragment PLA - forming hundreds of sub-millimetre sized microplastics. Although larvae showed a preference for leaf material when constructing cases, plastic use and fragmentation still occurred when leaf material was replete, indicating that this behaviour is likely to occur in natural environments which are polluted with plastics. This is thought to be the first documented evidence of active plastic modification by a freshwater invertebrate and therefore reveals a previously unidentified mechanism of plastic fragmentation and microplastic formation in freshwater. Further work is now needed to determine the extent of this behaviour across freshwater taxa and the potential implications for the wider ecosystem. This article is protected by copyright. All rights reserved. Environ Toxicol Chem 2022;00:0-0. © 2022 SETAC.