We investigated the breakdown of 2 leaf species, Croton gossypifolius (Euphorbiaceae) and Clidemia sp. (Melastomataceae), in a 4th-order neotropical stream (Andean Mountains, southwestern Colombia) using leaf bags over a 6-wk period. We determined the initial leaf chemical composition and followed the change in content of organic matter, C, N, and ergosterol, the sporulation activity of aquatic hyphomy cetes, and the structure and composition of leaf-associated aquatic hy phomy cetes and macroinvertebrates. Both leaf species decomposed rapidly ( k 5 0.0651 and 0.0235/d, respec- tively); Croton lost 95% of its initial mass within 4 wk compared to 54% for Clidemia . These high rates were probably related to the stable and moderately high water tempera ture (19 8 C), favoring strong biological a ctivity. Up to 2300 and 1500 invertebrates per leaf bag were found on Croton and Clidemia leaves after 10 and 16 d, respectively. Shredders accounted for , 5% of the total numbers and biomass. F ungal biomass peaked at 8.4 and 9.6% of the detrital mass of the 2 leaf species, suggesting that fungi contributed cons iderably to leaf mass l oss. The difference in breakdown rates between leaf species was consistent with the earlier peaks in ergosterol and sporulation rate in Croton (10 d vs 16 d in Clidemia ) and the faster colonization of Croton by macroinvertebrates. The softer texture, lower tannin content, and hi gher N content were partly responsible for the faster breakdown of Croton leaves. The rapid breakdown of leaf litter, combined with a low infl uence by shredders, is in accordance with previous findings. The high fungal activity associated with rapid leaf breakdown appears to be characteristic of leaf processing in tropical streams.