Solar ultraviolet radiation (UVR) is an environmental stressor that can have a variety of negative effects on aquatic organisms. The ctenophore Mnemiopsis leidyi is a highly transparent organism that has not been shown to actively avoid UVR or possess photoprotective compounds and may therefore be vulnerable to deleterious effects of UVR. Results of this study indicate that summertime UVR exposure equivalent to average UVR conditions within the top 0.5 m of the water column of the Rhode River, Maryland, USA, can cause mortality and reduced size of M. leidyi. Exposures tested did not, however, affect egg production. Experiments indicated a sharp threshold for the tolerance of M. leidyi to biologically effective UVR exposure. Mnemiopsis leidyi is an important component in many ecosystems; thus, changes in its abundance have the potential to significantly affect coastal and estuarine food webs and oyster, fish and sea nettle populations in systems like Chesapeake Bay.