SUMMARYFungal isolates from collections of cattle dung representing different stages in decomposition were examined for their ability biologically to modify the lignin and cellulose components of unamended native wheat straw (WS). Growth rate of fungal isolates on dung-extract agar was not correlated with their ability to affect biomass loss of WS. Later-sporulating colonists promoted greater biomass losses (total dry weight and cellulose). A late-appearing unidentified basidiomycete was most effective in degrading lignin and rendering the cellulosic components of natural lignocelluloses more susceptible to hydrolysis by cellulase. Fungal degradation of differentially14C-labeled natural lignocelluloses (LC) added to ground and pelletized WS was followed by monitoring14CO2evolution during a 48-da incubation. Three late-appearing species on dung were more efficient in producing14CO2from specifically cellulose-labeled lignocelluloses (LC*) than were three earlier-appearing species. The percentage of14CO2evolved from lignin-labeled lignocelluloses (L*C) was sixfold greater with the late-appearing basidiomycete than with any of the five ascomycetes. Present findings help to explain patterns of fungal development in cattle feces and the importance of lignin-degrading,“white rotting”basidiomycetes in feces decomposition.