Three enteric pathogens Yersinia enterocolitica 0:8, Salmonella typhimurium, and enterotoxigenic Escherichia coli, were examined for their ability to colonize granular activated carbon (GAC) in pure cultures and in the presence of autochthonous river water organisms. All three organisms readily colonized sterile GAC and maintained populations of ca. 10 to the 5th power to 10 to the 7th power CFU per. g. for 14 days when suspended in sterile river water. Exposure of pathogen biofilms on GAC to unsterile river water resulted in a gradual decline in pathogens on the carbon (0.08 to 0.14 log per day). When pathogens were introduced to sterile GAC in the presence of heterotrophic plate count organisms, they attached at levels similar to those in pure cultures and then decreased. When added with heterotrophic plate count bacteria to GAC supporting a mature biofilm of native river water bacteria, they attached at a lower level and decreased at a more rapid rate.