Plankton communities and physical variables in c. 5 km of the middle reaches of the Imo River, subject to sand dredging and oil/gas exploration, were surveyed in 2003 providing a preliminary checklist of plankton assemblages in relation to environmental parameters. Both phytoplankton and zooplankton were well distributed along the channel, with only minor differences between stations. Phytoplankton densities ranged between 21 and 397 cells l−1, with Bacillariophyceae dominant in terms of species richness and abundance. Among the zooplankton, Rotifera had the highest species richness but were less abundant than Copepoda and Cladocera. The impact of sand dredging and oil/gas exploration activities on the plankton assemblages appeared minimal.