We examined the efficacy of the photosystem II inhibitor 3-(3,4-diclorophenyl)-1,1-dimethyl urea (DCMU) for inhibition of algal photosynthesis in periphyton associated with submerged decomposing litter of Typha angustifolia. We also investigated the possible nontarget effects of DCMU exposure on heterotrophic microorganisms (i.e., bacteria and fungi) and extracellular enzyme activity associated with decaying litter. Standing-dead Typha leaf litter was submerged for 34 and 73 d, returned to the laboratory, and used for controlled laboratory experiments that examined the effect of DCMU on algal ([¹⁴C]bicarbonate, pulse-amplitude modulated fluorometry), bacterial ([³H]leucine), and fungal ([¹⁴C]acetate) production. Simultaneous assays also were conducted to examine the effect of DCMU on the activities of 4 extracellular enzymes (β-glucosidase, β-xylosidase, leucine-aminopeptidase, and phosphatase). DCMU significantly inhibited algal photosynthesis in light-exposed periphyton (p always < 0.0003), with strong inhibitory effects occurring within 5 min after exposure to DCMU. In contrast, DCMU had no significant direct effect on bacterial (p > 0.5) or fungal production (p > 0.3). Extracellular enzyme activities also were not significantly affected by exposure to DCMU. Heterotrophic microbial and enzyme activity assays were conducted in darkness to avoid any indirect effects of DCMU (i.e., heterotrophic responses to the inhibition of photosynthesis, rather than to DCMU itself). The apparent lack of nontarget effects of DCMU on heterotrophic microbial processes, combined with good efficacy against algal photosynthesis, suggest that DCMU may a useful selective inhibitor for investigations of interactions among litter-inhabiting microbiota.