Previous studies showed that harbors are suitable for development of toxic algae and could be a factor of their transport to other places. For the purpose of studying this possibility, samples collected bi-monthly from mid July 1997 to mid June 1998 from 2 harbors and a control station were analysed for the occurence of toxic and potentially toxic algae in relation to environmental factors.The results showed that nutrients are higher in harbor stations and showed great fluctuations.Also the percentage of toxic algae relative to total phytoplankton populations were higher and occured particularly during warm seasons. The following species, which produce potent toxins that can find their way through the food chain to humans, were relatively abundant:Alexandrium minutum occured in the period between July and September with maximum concentration of 50000 cells/L in July. Amphidinium carterae was present only in autumn with maximum concentration of 2673 cells/L in mid September. Gyrodinium cf.galatheanum was present in only one harbor in summer time with maximum at the end of July of more than 90000cells/L and absent in winter time. Pseudo-nitzschia pseudodelicatissima was only abundant in spring.Other species was also present but in small number such as: Alexandrium tamarense, Prorocentrum lima, P.minimum, P. mexicanum and Ostreopsis siamensis. Other species harmful to fish and invertebrates such as Gymnodinium mikimotoi and Chaetoceros spp. were also present.