Thirty-five clones, belonging to twelve species of dinoflagellates were isolated from planktons in brackish water ponds or from the wash-off epiphytes of Sargassum spp. These isolates were identified as species of Amphidinium carterae, A. klebsii, Gyrodinium instriatum, Coolia monotis, Gambierdiscus toxicus, Ostreopsis lenticularis, Prorocentrum lima, P. mexicanum, P. mininum and some unknown species belonging to Gonyaulax, Gymnodinium and Prorocentrum. Methanol extracts of the filtered cells were diluted with water to make 60% methanol solution and partitioned with n-hexane and chloroform. Both chloroform and aqueous fractions were screened for their toxicity in mouse (i.p. injection) and brine shrimp (solution). Chloroform extracts of G. toxicus, O. lenticularis, P. lima, A. klebsii and some clones of A. carterae and Prorocentrum sp. showed mouse toxicity. Brine shrimp larvae also responded to the methanol extract of these isolates in variable extents. Among them clone #2 and #4 of A. carterae, and clone #1 of A. klebsii were found the most toxic ones. It was also found that the chloroform extract of P. lima and G. instriatum exhibited toxicity to brine shrimp. Toxins belonging to different categories in these algal isolates were studied for their chemical nature. From cell cultures of one toxic P. lima clone eight okadaic acid (OA) analogues and derivatives were isolated. Among them, okadaic acid (OA), methyl ester of OA, dinophysistoxin-1 (DTX1), and two diol esters of OA, OA-D8 and OA-D9d, were identified by NMR and EI-MS analysis. One of the compound, OA-D9d was recognized as new in the literature. The remaining three compounds were preliminarily identified as new compounds of DTX4 type toxin and their structures remained to be determined. It was also noted that quite a lot of OA and DTX1could be isolated from the cell-free media of P. lima cultures. It seems that the free acid form of toxins are easily excreted outside the cells.