Abstract: The fish species assemblage and diversity of Majidun Creek, Lagos, Nigeria was investigated between January and December, 2010 to provide necessary baseline information that will help to maximize the sustainability of its fishery. Fish were collected with gill and cast nets. Associated environmental parameters such as water temperature and salinity were determined using mercury-in-glass thermometer and salinometer respectively. The specimens were sorted to the lowest taxonomic level and identified with identification manuals. Species diversity was determined by Simpson’s index while dominance and evenness by Shannon-Weiner index of diversity. The species assemblage comprised 517 individuals from 18 families, 8 orders, 20 genera and 23 species of fin and shell fishes. The dominant species were Sardinella aurita, Elops lacerta, Caranx hippos and Mugil cephalus with 256, 90, 59 and 29 individuals representing 49.52, 17.41, 11.41 and 5.61% of total fish catch respectively. The diversity indices’ estimates were Simpson’s Index (D) = 0.292, Simpson’s Index of diversity (1-D) = 0.708, Simpson’s Reciprocal Index (1/D) = 3.423, Shannon-Weiner Index (H) = 1.799, Shannon’s equitability (eH) = 0.574, Evenness (E) = 0.263 and Margalef Index (Dmn) = 3.52. Salinity and rainfall greatly influenced the species richness and diversity. High number of juvenile fish in this study is as indication that the creek served as a veritable spawning, breeding or feeding ground for freshwater and marine species. This study is an important contribution to fisheries of this creek where there is paucity of information concerning its fishes and fisheries.