Pesticides and fertilizers frequently detected in suburban streams could have an impact on leaf breakdown rates and fungal biomass in streams. In a field study comparing leaf breakdown rates and fungal biomass among suburban streams that differed in neighborhood socioeconomic status, significant differences in leaf breakdown rates were detected and related to stream velocity. Nutrient and fungicide (chlorothalonil and 4- hydroxy-chlorothalonil) concentrations did not differ among streams. Relationships between nutrient and fungicide concentrations and leaf breakdown rates were confounded by relationships with velocity. While breakdown rates are a good integrator of physical, chemical, and biological processes, it can be difficult to decipher which factor is having the greatest influence on differences in breakdown rates among streams. In a laboratory experiment, where the physical and chemical environment were controlled, the fungicides chlorothalonil, 4-hydroxy-chlorothalonil, and flutolanil inhibited biomass and sporulation rates of aquatic hyphomycetes and breakdown rates of tulip-poplar leaves (Liriodendron tulipifera L.).