This study aimed at estimating the Phytoplankton quotients; Bay, and to determine eutrophic levels of the bay. The study was conducted from April 2009 to April 2010. Nutrients and phytoplankton analyses were determined by spectrophotometric and microscopic techniques, respectively. Significantly higher (p<0.05) chlorophyll a concentrations were recorded during the dry season compared to the rainy season. At 57%, Cyanophyceae was the most abundant phyt by Chlorophyceae (28%), Desmidaceae (11%), Bacillariophyceae (4%) and Euglenophyta (1%). Among the Cyanophyceae, the most dominant species were Microcystis sp., Chroococcus sp., Anabaena sp and Cylindrospermopsis sp. Different phytoplankton patterns were observed between the offshore Maboko station and the inshore stations. The mean phytoplankton quotient for Kisumu Bay was estimated to be 4.1, indicating high eutrophic status. from anthropogenic activities, industrial and municipal wastes, and stricter enforcement of the established policies on the quality of discharges is recommended. There is also a need for environmental education and public awareness targeting the lake basin inhabitants to abate pollution in the lake.