Abstract: There has been no information on the phytoplankton community of the lower reaches of Okpoka Creek, Upper Bonny Estuary, Port Harcourt, Nigeria. This study was conducted to assess the species composition, diversity, abundance and distribution of phytoplankton. Phytoplankton and surface water samples were collected from May 2004 to April 2006 at mostly low tides from three stations according to standard methods. Phytoplankton was identified microscopically. Species diversity was calculated using standard indices. Surface water samples were analyzed for temperature, pH, transparency, dissolved oxygen, salinity, total dissolved solid, chloride, alkalinity, phosphate and ammonia. Phytoplankton community was made up of seven families: Bacillariophyceae (diatoms) were the most diversified and abundant algae with 26 genera and 63 species. Melosira varians (351 cell counts) was the dominant diatom. The examined physico-chemical parameters favoured the high abundance and population of phytoplankton in this creek.