The colony overlay procedure for peptidases (COPP) is a simple, fluorogenic assay that can rapidly detect and quantify Vibrio cholerae and Aeromonas hydrophila in well water. Cleavage of the substrate L-lysyl-7-amino-4-trifluoromethylcoumarin by enzymes present in Vibrio and Aeromonas species produces fluorescent foci on cellulose acetate membranes exposed to long-wave ultraviolet light. Vibrio cholerae O1, O139, O155, and A. hydrophila were readily detected using this procedure, whereas Enterobacteriaceae and other non-Vibrionaceae pathogens did not produce fluorescence. The assay is practical for assessing the relative safety of well water in areas that have experienced catastrophic devastation from natural disasters, acts of war, or civil strife and may help curb outbreaks of cholera and other enteric illnesses in affected areas. In tropical climates, the procedure may be adapted for use in areas without electricity.