The paper investigates whether the tertiarization and rapid urbanization faced by developing countries favour agglomeration economies. Focusing on Ecuadorian cantons, a productivity equation is estimated using the generalized method of moments model with instruments controlling for endogeneity. The varying impact of industrial concentration, diversity, competition and density across industries is investigated and, for the first time, the implication of the level of urbanization on agglomeration externalities is studied. Stronger effects are found for services. The threshold of urbanization at which diversity, density and competition agglomeration externalities all generate positive effects was 33%, while they seem challenged by congestion in highly urbanized cantons.