Full-text article. Also available at https://doi.org/10.1016/j.gexplo.2019.106413Investigation on accumulation of selected heavy metals of Zinc, Mercury, Copper, Lead, Chromium and Nickel in sediment, water and muscle tissues of Clarias gariepinus (African catfish) and Oreochromis esculentus (Singida tilapia) fish was done in Lake Rukwa, Tanzania. Samples were obtained from transects of 100 m long extending from Luika and Songwe River mouths to offshore. Water and sediment samples were collected directly from the study sites while fish were obtained from fisherfolk operating in the Lake. Sampling was done in dry and wet seasons. Heavy metals analysis was done using the Atomic Absorption Spectrophotometry. Concentration of heavy metals was higher in catfish than in tilapia (p < 0.05). There were no significant differences in metal concentration between seasons except for Zn (p < 0.05). In this study only Zn was above standard WHO concentrations in fish muscles. Likewise, the concentrations of heavy metals were within recommended limits in water except Pb. The detected metals in sediment were above recommended limits. Other heavy metals in particular Hg, Ni and Cr were not detected in all samples. Therefore, studied fish from Lake Rukwa may threaten human health upon consumption. The detected heavy metals in water were within the maximum residual levels (MRLs) permitted by WHO. Sustainable Lake Rukwa's fish, ecosystem management and conservation are recommended to discourage heavy metals discharge from elevating beyond permissible limits and thus prevent harmful health effects to fish consumers and water users.