From February to May 1998 there was a continuous algal bloom in the Port Shelter, an inlet of an enclosed bay in the northeast Hong Kong\'s coastal waters. Phytoplankton data collected from a permanent station located in this area was analysed. Diatoms and dinoflagellates were the two main groups which dominated the phytoplankton. In general, when there was an increase in the density of diatoms there was a decline in the density of dinoflagellate, and vice vera. Dinoflagellate species which included the fish killer Gyrodinium digitatum, started to bloom in late February and reached their highest density on March 18, when fish kills were first reported. During the next 16 weeks, dinoflagellate species dominated twice, in mid-February and mid-March. Hydrographic and climatic data suggest that the dinoflagellate bloom might have been triggered by stormy weather and the Kuroshio Current, which flows into Hong Kong from the southeast.