The abundance and distribution of phytoplanktons of Bahoo-Kalat estuary at southeast of Iran were studied from December 1996 to November 1997. Samples were collected from different depths of 8 statios with 1 Lit. Nansen water sampler The mean yearly density of phytoplankton was 8825±1955.The minimal and maximal monthly densities were sequencely observed in December and October, significantly different from the other months by HSD Tukey test. Concentration of phytoplankton in stations 4 to 7 was more than the other stations without any significant different between the stations by HSD Tukey test, and between different depth layers with t-test. Phytoplankton abundance was greatly increased from Winter to pre-monsoon, monsoon and post-monsoon. Bloom of Gloeotrichia was seen in October at the ending of monsoon. Monsoon probably has a great influence on the distribution and abundance of phytoplanktons of Bahoo-Kalat estuary.