The increasing demand for fish as dietary protein source has led to overfishing, use of banned fishing devices and other activities that are inimical to fisheries and threats to aquatic systems reducing their productivity in general. This study was carried out to identify and estimate the impact of gill net selectivity on Schilbe mystus (Linnaeus, 1758) in lower River Ogun, Akomoje. 102 samples of different sizes were randomly selected from catches made with the use of gill nets of different mesh sizes (3-8cm) between June and November 2017. Length and weight parameters were estimated by standard method. Abundance was least in the months of September and October which was their spawning season. The highest represented length class was from 16-16.9 cm and 14-14.9 cm, accounting for 16.7% and 14.7% of the average monthly catches respectively. There was gill net selectivity with the smallest mesh size gill net being the most efficient, accounting for 95% of the number of the total catch. The small and medium gill nets were largely inefficient compared to the ‘very small' gill net.