The gamete-specific effects of UV radiation on fertilization in the invasive freshwater zebra mussel (Dreissena polymorpha) are addressed. Both eggs and sperm of zebra mussels were highly sensitive to UVB. Sperm irradiated with 118 mJ/cm² UVB showed decreased fertilization rates when inseminating nonirradiated eggs. Sperm binding was unaffected; however, acrosomal integrity and sperm incorporation into the egg cytoplasm were reduced by UVB irradiation. There was a decrease in first zygotic cleavage. The level of UVB needed to inhibit fertilization was lower than that required for inhibition of sperm motility. UVB-irradiated eggs also showed reduced function when inseminated by nonirradiated sperm. UVB-induced damage in eggs most commonly resulted in a stoppage or delay of meiosis. The levels effective in killing zebra mussel larvae should be sufficient to inhibit fertilization.