Clarias gariepinus and Oreochrornis niloticus are two hot-water fish species of African origin that have been recently reported to occur unnaturally in hot-water resources and springs in the Sakarya River Basin (northwestern Turkey). Within effective planning strategies to maintain native biodiversity, monitoring fishes' well-being can be useful to evaluate the status of nonnative fish populations and their degree of adaptation to new environments. Relative weight (W-s) is a condition index widely used to estimate the well-being of fish by comparing the actual weight of a specimen with a standard weight (W-s) that is the ideal weight of that fish at that length. In this study, length and weight data of C. gariepinus and O. niloticus, collected throughout the Sakarya River Basin, were used to estimate W, equations. The resulting W-s equations were log(10)Ws = -3.668 + 1.885 log(10)TL + 0.2087 (log(10)TL)(2) (TL range: 18-45 cm) for C gariepinus and log(10)W(s) = -8.796 + 6.751 log(10)TL - 0.8479 (log(10)TL) (TL range: 8-28 cm) for O. niloticus. The use of these W-s equations to estimate the fishes' condition is suggested as a monitoring tool to assess the status and the best management actions of these two nonnative species populations throughout the Sakarya River Basin.