Cyanobacteria (blue-green algae) and actinomycetes are microorganisms in catfish ponds that produce compounds that render catfish off-flavor. Karmex has been suggested as an algicide to help eliminate this problem. Diuron, the active ingredient in karmex, and its major degradative product, 3,4-dichloroaniline (3,4-DCA) were employed to determine the effect of Karmex on the catfish pond ecosystem. The objectives of this research were to determine the effects of diuron and 3,4-DCA on cyanobacteria and geosmin-producing actinomycetes from catfish ponds; microbial processes involved in the carbon, nitrogen and sulfur cycles; several bacterial pathogens of catfish and the fate of diuron and 3,4-DCA in catfish ponds. Neither compound had any obsevable effect on the growth or geosmin-production by actinomycetes. Microbial degraders were not adversaly affected by exposure to the diuron or 3,4-DCA, while lipid degraders were stimulated by the presence of diuron and unaffected by 3,4-DCA. Ammonification, nitrate reduction and denitrification, likewise, were not adversely affected by diuron nor 3,4-DCA. Nitrification was inhibited at diuron concentrations higher than 5 mg/L. Sulfide production was not increased by the presence of diuron or 3,4-DCA. Edwardsiella ictaluri, Pseudomonas fluorescens, Aeromonas hydrophila, and Aeromonas salmonicida, four common bacterial pathogens, were inhibited by diuron and 3,4-DCA. Diuron and 3,4-DCA were rapidly degraded by catfish pond microorganisms.