Measurements of primary productivity can improve assessment of the fish yields from tropical lakes. In tropical African and Indian lakes commercial fish yields increase logarithmically as primary productivity increases arithmetically. The regression equation describing the relation between fish yields (FY) and gross photosynthesis (PG) for eight African lakes is log FY = 0.113 PG + 0.91. The coefficient of determination is 0.57. The regression equation based on fifteen tropical Indian lakes, log FY = 0.122 PG + 0.95, corroborates the relation for Africa.