An assessment was carried out on the impact of Lernaea cyprinacea on fish populations ten years after its first outbreak in the Malilangwe reservoir, and Lernaea cyprinacea is currently showing no sign of declining in the reservoir. Eight fish species were examined for ectoparasite prevalence and intensity. The possible relationship between L. cyprinacea infestation and environmental factors were investigated. Two parasite species, L. cyprinacea in Oreochromis mossambiccus, Oreochromis placidus, Oreochromis macrochir, Labeo altivelis and Tilapia rendalli and trematode cysts (Clinostomoides brieni) in Clarias gariepinus were found. Lernaea cyprinacea prevalence was 100% amongst all cichlids but varied for L. altivelis. Parasite intensity increased during the cool, dry season (May to July), with the greatest mean intensity being observed amongst the cichlids. There was a significant relationship between parasite intensity and environmental factors; dissolved oxygen (p < 0.05), temperature (p < 0.001) and pH (p < 0.001).