The phytoplankton of a tidal creek in Lagos was investigated for six months (September 1998-February 1999). The physico-chemical environment was essentially neutral (pH 7.00-7.30), relatively more transparent (> 0.23 m), expressed higher conductivity (> 0.10 mS cm(-1)), higher chloride values (> 23.48 mg L(-1)), lower nitrate content (< 0.70 mg L(-1)), lower sulphate values (< 0.45 mg L(-1)) and phosphate values (< 0.12 mg L(-1)) in the dry months. In the wet months, September to November 1998, conductivity (< 0.16 mS cm(-1)), transparency (< 0.80 m), salinity (< 0.04%), dissolved oxygen (< 24.00 mg L(-1)), chloride (< 24.21 mg L(-1)) and phosphate (< 0.21 mg L(-1)) were recorded. The phytoplankton was dominated by diatoms (48 pennate forms and 23 centric forms). The other algal groups comprised six blue- green algae and six green algae. Species richness (d) was high in stations nearer the Lagos lagoon for the six sampling periods. Shannon-Weaver index (H1) was lowest in November 1998 and highest in October 1998 and February 1999 while evenness of species J was low (< 0.71) throughout the period. Higher phytoplankton biomass was recorded in the dry months than wet months.