Study of gonads of the mesopelagic ctenophoreBathocyroe fosterirevealed that the animals are dioecious for at least part of their life span. Specimens were collected using the Johnson Sea-Link I and II submersible vehicles during the periods of October 4–11, 1988, off the Bahamas Banks and July 18–20, 1991, off Cape Hatteras at depths of 550–900 m. Each animal was provisionally sexed by scanning all comb rows with a dissecting microscope. The provisional sex was confirmed after preparation of squashes of ad-tentacular and ab-tentacular comb rows for eggs and sperm at various magnifications using dark field optics and histological sections. With two exceptions, only one type of gamete was found in any sexual specimen and all comb rows bore the same type of gamete on both sides of the gastrovascular canal. UnlikeOcyropsis, the only other dioecious, planktonic ctenophore known,Bathocyroehas a gonad location typical of all other ctenophores. Our observations suggest that ctenophore dioecy is not restricted to the epipelagic, oceanic habitat and is not a property of a single genus with atypically located gonads. Dioecy has apparently evolved secondarily at least twice in habitats largely occupied by a group of related ctenophores which are all simultaneous hermaphrodites.