Composition and density of phytoplankton in mangrove forest at Sikao District,Trang Province, was determined from May 1996 to May 1997. The objectives of thisstudy were to elucidate the spatial and temporal variation in phytoplankton communityand to establish the relationship between plankton diversity and abundance andenvironmental parameters. Phytoplankton sampling was carried out every other monthfrom 6 different locations along Klong Sikao. Phytoplankton samples were collected atthe in-coming tide and the stagnant hightide. Salinity, temperature, pH,transparency, depth, dissolved oxygen were recorded in situ and water samples werecollected for further analyses of phosphate, nitrate and nitrite, silicate,chlorophyll_a, total organic carbon and total organic nitrogen. Sixty-two genera of phytoplankton from 5 classes 3 divisions found in mangrovearea at Klong Sikao consisted of 47 genera of diatoms, 7 genera of dinoflagellates, 4genera of green algae, 3 genera of blue-green algae and I genera of silicoflagellate.The highest number of 56 genera of phytoplankton was recorded in rainy season ofAugust 1996 and the lowest number of 40 genera was found in dry season of December1996. Maximum density of phytoplankton of 202,319 cells/l occurred in dry season inDecember 1996 and the minimum density of 40,632 cells/l occurred in this same seasonin March 1997. Phytoplankton found in abundance were diatoms

~iGuinardia~i sp.,~iThalassionema~i spp., Rhizosolenia spp., ~iThalassiosira~i spp., ~iThalassiothrix~i spp. and~iCyclotella~i spp. and blue-green algae

~iAnabaena~i sp. and ~iOscillatoria~i spp. Variation indiversity and abundance of phytoplankton in Klong Sikao showed the tendency to relatewith temperature, salinity and the concentration of nutrients. This result indicatesthe pristine and productive nature of Klong Sikao mangrove based on the highdiversity and abundance of phytoplankton.