Alexandrium minutum containing PSP toxin is a harmful dinoflagellate. We have carried out an toxic algal monitoring program along the south west coast of Taiwan since 1992 to understand its occasional blooming and the correlation with toxic clam. This report was based on the data collected from 1992 through 1995. Waters of the clam rearing ponds, neighboring fish or crustacean ponds and the ditches between them from area of Changhwa, Yunlin, Chiayi, Tainan, and Pingtung Hsien, were collected. Physical data and A. minutum of the water samples were studied. Total number of the water samples were 1091. Among them 205 samples (19%) had cells of A. minutum in it. Toxic algae were found existing through the year, but more frequently (26%) found in December. Among five Hsiens the highest occurrence (97%) was found in Pingtung. Samples containing A. minutum were mostly found in one of the ditches near the estuary of Kaoping river. Mostly, the cell density was less than 10 cells/ml, however, a few water blooms containing algal cells more than 10,000 cells/ml were found, the culture milkfish and tilapia were morbid. Salinity of the water samples containing A. minutum ranged between 5-36 ppt with the highest occurrence of algae between 11-20 ppt. Water temperature ranged between 14.6-36℃ with the highest between 20-30 ℃. pH ranged between 7.38-10.27 with the highest between 7.51-8.00. The nutrient concentrations were NH4-N 80-2700 ppb, NO3-N 33-259 ppb, NO2-N 1-333 ppb, PO4-P 8-633 ppb and SiO3-Si 190-4310 ppb, the range of samples containing A. minutum was narrower, however, there was no relationship between the algal density and the nutrient concentration.