Abstract: Twenty five fish species representing 16 families were recorded in the Ologe Lagoon, South-West Nigeria. Adult stages of 12 of them were freshwater (48%), 4 were estuarine (16%) and 9 were near-shore marine species (36%). The slight seasonal variation in the catch composition of the fish fauna of the lagoon was attributed to the slight seasonal fluctuation in salinity marked with higher rainfall volumes in the rainy season than in the dry. Availability of food organisms was also noted for the seasonal distribution of some of the fish species. Temperature and pH varied slightly, while DO was higher in the dry season but lower in the rainy season. Distance from salt water influence and much river influx with the associated reduced salinity appear to be the major meteorological forcing operating in the lagoon. The lagoon is very low brackish water. The highest species richness was observed in September with 13 species, while the lowest with least abundance was observed in August (when there was a break in rains) with 7 species. The most abundant species during their occurrence were T. mariae (10.1%), C. senegalensis (9.84%) and M. electricus (7.51%). Species richness and total abundance were found to increase slightly during the rainy season than the dry season. Availability of much juvenile forms indicated that the lagoon forms a good nursery ground for many fish species.