Microplastic fibers (MPF) are a ubiquitous marine contaminant, making up to 90% of global microplastic concentrations. Imaging flow cytometry was used to measure uptake and ingestion rates of MPF by blue mussels (Mytilus edulis). Mussels were fed a diet of Rhodomonas salina and MPF concentrations up to 30 MPF mL−1, or 0.374% of available seston. Filtration rates were greatly reduced in mussels exposed to MPF. Uptake of MPF followed a Holling's Type II functional response with 95% of the maximum rate (5227 MPF h−1) occurring at 13 MPF mL−1. An average of 39 MPF (SE ± 15, n = 4) was found in feces (maximum of 70 MPF). Most MPF (71%) were quickly rejected as pseudofeces, with approximately 9% ingested and <1% excreted in feces. Mussels may act as microplastic sinks in Gulf of Maine coastal waters, where MPF concentrations are near the order of magnitude as the experimental treatments herein. •Mussels take up MPF in a quantifiable and predictable manner.•MPF depress the mussel's filtration rate of microalgal prey at levels >3 MPF mL−1.•Most MPF within the body resided in the digestive gland, with less in the gill.•MPF are lost through pseudofeces and feces, with 60% of body load gone in 9 h.