This study reports length–weight relationships for 10 species of fish encountered in the only two freshwater lakes wholly in the Rift Valley of Kenya, Naivasha and Baringo. In Lake Naivasha, none of the species analysed was native to the lake. Two were introduced species non-native to East Africa (Micropterus salmoides, Cyprinus carpio), with three native to East Africa but not to the lake (Tilapia zillii, Oreochromis leucostictus, Barbus paludinosus). In Lake Baringo, all species were native (Oreochromis niloctictus baringoensis, Labeo cylindricus, Barbus intermedius autralis, Barbus lineomaculatus, Clarias gariepinus). Specimens were collected by gill netting in September 2004 and October 2005. With the exception of B. lineomaculatus, the b values in the relationship W = aLb were between 2.90 and 3.22.