Estimativa da biomassa de raizes de duas espécies de Cecropia e sua relação com a associação ou não das plantas a formigas This paper proposes the hypothesis that ants associated with several species of Cecropia, besides their other possible role as allelopathic agents (sensu Janzen, 196S). are concentrating nutrients for the plants. This hypothesis was tested by comparing the development of the biomass of the root system through the ratio fresh root biomass/fresh stem biomass of an ant-associated species, C. concolor Willdenow, and an ant-free species, C. sciadophylla Martius. The results indicate that C. concolor has a mean ratio root/stem of 0,27 (n = 5), whereas in C. sciadophylla the same ratio is 0,50 (n = 5). a difference significant at the 5% level This trend, therefore, corroborates the initial hypothesis.