Aims: To investigate changes in the bacterial population and the safety of the biofilm at the end of the drinking water distribution system in Seoul (Korea), selective media and bacterial community analyses were applied to a semi-pilot galvanized iron pipe (GIP) model. Methods and Results: No total coliforms or faecal streptococci were detected on m-Endo or m-Enterococcus agar. No Salmonella spp. and Shigella spp. were detected on bismuth sulphite agar or Hektoen enteric agar, respectively. The latter two media detected coliforms, where m-Endo was negative. Biofilm formation started within 1 week (ca 10(4) CFU cm(-2)) and exceeded 10(5) CFU cm(-2) within 6 weeks. Although the fatty acid methyl ester analysis revealed dynamic changes in bacterial composition, Micrococcus , Bacillus , and Pseudomonas spp. were persistent members of the biofilm community. Micrococcus spp. was detected most frequently and in high numbers. Conclusions: Coliforms and Enterococcus species can be recovered from biofilms in water distribution systems. Significance and Impact of the Study: This study illustrates the role of biofilms in the chronic deterioration of the water-distribution system in Seoul (Korea).