ABSTRACT. – We report on the composition, structure, abundance, biomass variations of the coastal phytoplankton communities and their relationship with respect to environmental parameters in the Ebrié Lagoon, south-eastern Ivory Coast. The studies reveal the environmental parameters reflecting seasonal changes related to rainfall distribution pattern and tidal seawater incursion. Surface water temperature (26.3-31.1 °C), pH (6.54-8.11), transparency (0.15-1.7 m), salinity (0-27.5), SRP (0.04-2.76 μmol L-1) show increased values in the dry compared to the wet season. On the other hand nitrate (0-21.56 μmol L-1), SRSi (20.72 to 143.31 μmol L-1) show higher values in the wet season. The 122 identified species were represented mainly by Bacillariophyta (36.1 %), Chlorophyta (23.8 %), followed by Cyanoprocaryota (22.1 %) and Euglenophyta (14.7 %). Species composition of phytoplankton was typical of eutrophic conditions and was frequently characterized by the presence of Bacillariophyta. Phytoplankton standing crop attained the highest counts during short or long dry season (19-500 x 106 cells L-1) due to the dominance of Cyanoprocaryota species (Phormidium sp., Cylindrospermopsis raciborskii, Oscillatoria tenuis, O. limosa, Lyngbya martensiana and Microcystis aeruginosa). The Shannon-Wiener and Equitability indices indicated pollution stress and dominance by a few species. Recorded chlorophyll a values give the Ebrié Lagoon the eutrophic status. It is suggested that increasing tidal influence associated with reduced rain events may have enhanced elevated salinities and created conditions for the development of more algal cells, hence higher chlorophyll a records