Although ivermectin is a widely used lipophilic parasiticide, data on its potential bioaccumulation in aquatic invertebrates are scarce. In this study, bioaccumulation patterns of radiolabeled (3)H-ivermectin from sediments into tissues of the sediment-dwelling worm Lumbriculus variegatus were investigated and assessed. Two independent studies are described. In the first study, bioaccumulation kinetics of ivermectin in L. variegatus were investigated by following uptake, elimination, and biotransformation of the compound in worms exposed in artificial sediment. In the second study, possible effects of sediment characteristics on bioavailability of ivermectin were evaluated by comparing accumulation in worms exposed in three different natural sediments and one artificial sediment (organic carbon content 2% to 21%). During 28 days of exposure, ivermectin accumulated in worms exposed in all sediments with calculated bioaccumulation factors ranging from 0.2 to 11.0. Bioaccumulation factors correlated with total increase in biomass of worms during the exposure period and with organic carbon content of the sediments. Calculated biota-sediment accumulation factors ranged from 2.1 to 16.6, indicating that other variables, like quality of organic carbon and feeding behavior of the worms, may have influenced the bioavailability of ivermectin. Biotransformation products of ivermectin were only found in water, but not in sediment and worms. Results show that ivermectin has potential to bioaccumulate in L. variegatus, which could indicate a risk for biomagnification of the compound in the food chain.