In April-May a mass occurrence of Chattonella aff. verruculosa (Rhaphidophyceae) caused brownish sea in parts of the Skagerrak and the North Sea. Both wild fish and fish in pens were killed during the bloom. In April, Chattonella grew to a dense population in an area to the north of Skagen in the Skagerrak. On April 12 it was also common outside the Limfjord at the west coast of Denmark. In the very beginning of May (1-3) significant mortality among large Atlantic salmon in pens were reported by fish farmers at the south-west coast of Norway. Later in May (7-19) very high concentrations of Chattonella were recorded all along the west coast of Denmark, from the Jammerbukt and at least to the Esbjerg-area. This could partly be a spreading from the Skagen-area via an unusual, temporary south-west current, but in situ growth of algae seen along the west coast of Denmark in April could have contributed as well. The bloom developed in water bodies rich in nitrate, most probably brought to the Skagen-area from the southern North Sea. High N:P atomic ratios in particulate material, mainly consisting of Chattonella, collected by the end of the bloom, pointed to phosphor-limitation of the bloom. The number of common zooplankton collected by net (180 µm) in the blooming area was small. This may have been due to zooplankton avoiding the Chattonella bloom, which again may have led to a low grazing-pressure. This was, to our knowledge, the first record of this species in European waters, and world-wide, the first report on fish mortality associated with this species.