Synthetic microplastic fibres (MPFs) from textiles constitute a major proportion of microplastic pollution in marine systems, having been frequently reported in all environmental compartments, including the digestive systems of many types of biota. Despite their widespread distribution, the direct interactions of MPFs with biota remain poorly understood. This study compares ingestion and egestion of acrylic fibres in the green sea urchin Strongylocentrotus droebachiensis. For reference, natural wool fibres were also studied. In addition to 'pristine' acrylic and wool fibres, 'aged' fibres exposed to natural seawater over two weeks were also used to study the influence of biofouling on ingestion rates. Fibres of 1-5 mm length were dispersed in the incubation seawater at concentrations of approximately 43 fibres per mL and sea urchins were then added and exposed individually for 48 hours. All fibre types (wool fouled and unfouled; acrylic fouled and unfouled) were readily ingested and subsequently egested with the faeces. No negative effects of fibre ingestion on the urchins were recorded. High numbers of fibres were counted in faecal pellets, suggesting that the fibres pass through the gastrointestinal tract and leave the organisms packaged in faecal material. The implications of biofouling on fibre uptake rates in benthic grazers and the fate of microplastic fibres upon repackaging in faecal matter are discussed.