HABs have frequently occurred in Hong Kong in the past 20 years. There were total 740 cases of HABs during 1983-1998. We mapped all occurrences of HABs using a GIS system to examine spatial distribution of HABs in the Hong Kong territorial waters during 1983-1998. The mapped spatial distribution of HABs was compared with fish farm zones, distribution of beaches or topographic features. The comparisons revealed that places of HABs occurrences coincided with fish farm zones, public beaches, or shallow water zones (<10 m). Most HABs occurred in sheltered waters such as Tolo Harbour and Porter Shelter to the east of Hong Kong; and >50% of total HABs cases were between Feb and May with over 10% of total each month and the least frequency was during July-November, under 5% each month. The 7 most frequently-occurring species including (in descending order) Noctiluca scintillans, Gonyaulax polygramma, Skeletonema costatum, Mesodinium rubrum, Prorocentrum minimum, Ceratium furca, and Prorocentrum triestinum were also mapped. The first 4 species were found to occur in wide regions while the next 3 species only occurred in the northeastern waters where oceanic waters dominate most of time. Noctiluca scintillans blooms occurred mostly during December to May, and Gonyaulax polygramma blooms only between February and May when oceanic conditions prevail. Skeletonema costatum blooms were mostly between May and September. Due to rainfall and the Pearl River estuarine plume influences, salinity started to decrease during this period. These spatial and temporal distribution patterns of HABs were believed to be related to environmental factors such as winds, rainfall, temperature, salinity, and nutrients.