The valuation of biodiversity and environmental resources is important and of benefit to understanding the healthy functioning of the environment.  Diversity of fishes in Asejire Lake, Nigeria was studied for a period of seven months, from February to August, 2014.  The lake was naturally divided into two strata--upper and lower reaches--as there was one landing site in each stratum.  Fish catches from fishermen were assessed, sorted into taxonomic categories, counted and weighed. Diversity indices package and cluster analysis were used for the statistical analysis.  Twenty-four fish species from fourteen families were identified during the study period.  Chrysichthys nigrodigitatus was the most abundant species by number (46.6%) and weight (34.3%) while Gnathonemus senegalensis was the least observed in the catch.  Bagrid (46.7%) and cichlid (46.3%) families had the highest contribution by abundance and weight respectively.  The greatest number of taxa was recorded in May and least in February.  The smallest Simpson diversity index (0.50) was observed in February and largest in July (0.88).  Simpson diversity index for the lake (0.70) was observed to be high with low dominance (0.30) and evennesss (0.21).  Cluster analysis by hierarchical classification of the identified species showed six distinct groups (Synodontis batensoda, Tilapia mariae, Hydrocynus forkalis, Labeo coubie, Pelmatochromis guentheri and Polypterus senegalensis) of fish assemblages at 95% similarity level.  Abundance-biomass comparison (ABC) curves revealed that Asejire Lake is a disturbed aquatic ecological system.  The disturbance might be attributed to over-exploitation of the fish resources in the lake and the waste discharge into the system from the industry located in the area.  This may have negative implications on the fish production and conservation of the diverse species in the lake.  Therefore, there is need for monitoring and proper management of the lake system for sustainable fish production and conservation for the posterity.