The rotifer Brachionus plicatilis is a very important live feed for rearing larvae of finfish, prawn and crab. Its biomass, morphology and occurrence in culture ponds in the Southern part of Taiwan Tungkang area were investigated to understand its natural resource and ecological conditions for growth. In September 1990 to August 1991, the rotifer was often found in brackish culture ponds with densities less than 10 individuals/ml. Three types of the rotifer were distinguished according to their size and morphology; the occipital margin of the lorica in L-type was blunt, while in S- and SS-type, it was acute and excavated at the point of projections in SS-type, and their lorical sizes were 162-243 痠, 150-205 痠 and 94-163 痠, respectively. L-type occurred during the low temperature season; S-type, all the year round; and SS-type, high temperature season. The optimum culture conditions for SS-type were at a salinity of 10-20 ppt, and temperature of 30-33 |C, suitable feeds were Tetraselmis chui, Nannochloropsis oculata and Isochrysis galbana. There was no significant difference in sizes of SS-type cultured at different temperaturs and salinities, and by different algal items.