Calliopaea bellula, a sacoglossan opisthobranch, was recently found in the Ria Formosa, a coastal lagoon in southern Portugal feeding upon eggs of the cephalaspidean opisthobranch Haminoea orbignyana, the second most abundant gastropod inhabiting the sea grass habitats in the lagoon. The feeding behaviour of C. bellula is described together with photographic images of this process. Egg-masses of other common opisthobranch species collected in the Ria Formosa were also offered, but they were always rejected. This is the first known case of predation on eggs of H. orbignyana.