A new virus from the Crab Carcinus maenas has been found in the nuclei of conjunctive and blood cells. Its localization, morphology, size, morphogeny relate it to the Baculovirus group. By some aspects it appears to be related closer to the insect viruses from Oryctes rhinoceros, and to red mite viruses from Panonychus ulmi and P.citri than to the nuclear polyhedrosis of the pink shrimp Penaeus duorarum.