Aims: To evaluate the occurrence, identity and antimicrobial resistance of Gram-negative bacteria isolated from municipal water supplies, treated water, and dialysate of all 85 Greek haemodialysis centres. Methods and Results: A total of 141 Gram-negative bacterial isolates (98 non-fermentative and 43 enterobacteria) were recovered from 255 water samples. Twenty-four of them were isolated from tap water, 31 from treated water, and 86 from dialysate samples. The mean concentrations (CFU per 100 ml +/- S.D.D.) of the positive Gram-negative bacteria samples were 69.2 +/- 43.9, 31.2 +/- 28.7 and 3552.3 +/- 4485'0, respectively. The most common isolates, in order of frequency were Pseudomonas aeruginosa (22.7%), Chryseobacterium meningosepticum (14.9%), Stenotrophomonas maltophilia (13.5%), Escherichia coli (12.8%) and Enterobacter cloacae (7.8%), representing 71.6% of all isolates. Ps. aeruginosa was the most prevalent isolate in all types of water sample followed by C. meningosepticum in tap and treated water and by E. coli in dialysate. Nineteen per cent of the enterobacteria and 35% of the non-fermenters were resistant against three or more of the nine antibiotics tested. Conclusions: These data suggest that dialysate and treated water could be a source of infection for several non-fermentative and enterobacterial species. Impact of the Study: Microbiological monitoring of such samples is needed in order to know the identity and antibiotic resistance profiles of their potentially pathogenic bacterial population.