Calliopaea oophaga feeds on the eggs of other opisthobranchs. It pushes its head through the mucus coating of the food egg mass, dragging the remaining part of the body inside by muscular contractions. During feeding the head is ‘inflated’ by displacement of hemolymph, causing the eyes to appear mediodorsal as in the genus Costasiella. C. oophaga forms courtship groups, but usually only two animals in a group are in the act of copulation at any one time. C. oophaga has an extremely long, thin penial style, which is waved about vigorously before being inserted through the vaginal opening of the partner. During spawning the distal part of the oviduct swells up. The eggs are irregularly shaped at the time they leave the oviducal opening, but assume their regular spherical shape shortly after. The egg capsules are almost spherical and contain an opaque ‘albumen’, which turns clear before the first cleavage. The eggs are arranged in a loosely coiled helix inside the often regularly spirally coiled egg mass. The taxonomic position and familial affiliations of C. oophaga are discussed.