An active commercial fishery for L. Naivasha started in 1959 using gill nets for tilapias and rod and line for the largemouth bass. Between the 1970s and 2000 the major fishery fish species landed were Oreochromis leucostictus (Trewavas, 1933), Coptodon zillii (Gervais, 1848) and Micropterus salmoides (Lacepede, 1802) (Muchiri & Hickley, 1991; Hickley et al., 2002). However, the lake originally contained only one species, the endemic Aplocheilichthys antinorii (Vinc) which was last recorded in 1962 and is believed to have disappeared from the Lake. Since 1925, there have been numerous (over 10) introductions of various fish species, both of commercial and non-commercial value. However, some of the introductions are also suspected to have been from intruders from the inflowing rivers. A comprehensive history and chronology of species introduction in Lake Naivasha is provided by Njiru et al. (2017). Presently, there are nine fish species that support the fishery of Lake Naivasha. These include: Nile Tilapia (Oreochromis niloticus (Linnaeus, 1758), the Blue-spotted tilapia (Oreochromis leucostictus), Red-bellied Tilapia Coptodon zillii: (Gervais, 1848) (formerly Tilapia zilli), largemouth bass (Micropterus salmoides), Louisiana red swamp crayfish (Procambarus clarkii (Girard, 1852), river cyprinid Barbus paludinosus (Peters, 1852), common carp (Cyprinus carpio) and the African sharp tooth catfish (Clarias gariepinus) (Burchell, 1822)). Knowledge of the size selectivity of fishing gear types is crucial to fisheries management and ecology in order to maximize sustainable use (Huse et al., 2000: Emmanuel et al., 2008). Gear selectivity studies in Lake Naivasha are not well documented necessitating the need for a comprehensive research on this aspect. Particularly, gill net selectivity effects on O. niloticus which is the latest introduction and most preferred species (Waithaka et al., 2015b) are not understood. The purpose of this study was to assess gillnet selectivity on the commercially important fish species of Lake Naivasha. READ MORE