Survival, longevity, and herbivory patterns of two leaf cohorts of two pioneer tree species, Cecropia obtusifolia and Heliocarpus appendiculatus, growing as seedlings in natural canopy gaps, were studied in the rainforest of Los Tuxtlas, Mexico. The leaf cohorts of C. obtusifolia did not differ in longevity and mortality pattern, and the leaf populations experienced low levels of herbivory. Herbivory was higher in H. appendiculatus than in C. obtusifolia; in H. appendiculatus, also, the percentage of leaf area lost per day (herbivory rate) was higher for young than mature leaves. The rates of herbivory in C. obtusifolia at Los Tuxtlas are lower than those reported for the same species in Panama; while the herbivory rates of H. appendiculatus are the highest reported for any pioneer neotropical species. The two leaf cohorts of H. appendiculatus differed in their longevity since an episode of intense and sudden mortality of leaves occurred when herbivory approached 70% of leaf area damaged, regardless of the age of the leaves. It is suggested that the mortality pattern and longevity of leaves of seedlings and saplings of H. appendiculatus are determined by herbivory but this does not appear to be so in C. obtusifolia.