ATP and ergosterol were compared as indicators of fungal biomass associated with leaves decomposing in laboratory microcosms and streams. In all studies, the sporulation rates of the fungi colonizing leaves were also determined to compare patterns of fungal reproductive activity with patterns of mycelial growth. During leaf degradation, ATP concentrations exhibited significant, positive correlations with ergosterol concentrations in the laboratory and when leaves had been air dried prior to being submerged in a stream. However, when freshly shed leaves were submerged in a stream, concentrations of ATP and ergosterol were negatively correlated during degradation. This appeared to be due to the persistence of leaf-derived ATP in freshly shed leaves during the first 1 to 2 weeks in the stream. Estimates of fungal biomass from ergosterol concentrations of leaf litter were one to three times those calculated from ATP concentrations. ATP, ergosterol, and sporulation data generally provided similar information about the fungi associated with decomposing leaves in streams during periods when fungi were growing. Ergosterol concentrations provide a more accurate indication of fungal biomass in situations in which other organisms make significant contributions to ATP pools.