A virus infecting the haptophyte Phaeocystis pouchetii (Hariot) Lagerheim was isolated from Norwegian coastal waters in May 1995 at the end of a bloom of this phytoplankter. The virus was specific for P. pouchetii because it did not lyse 10 strains of P. globosa Scherffel, Phaeocystis sp., and P. antarctica Karsten. It was a double-stranded DNA virus, and the viral particle was a polyhedron with a diameter of 130–160 nm. The virus had a main polypeptide of about 59 kDa and at least five minor polypeptides between 30 and 50 kDa. The latent period of the virus when propagated in cultures of P. pouchetii was 12–18 h, and the time required for complete lysis of the cultures was about 48 h. The burst size was estimated to be 350–600 viral particles per lysed cell.