The sources of gentamicin resistant pseudomonads and enterobacteria were studied in detail. A total of 1703 gentamicin resistant gram negative bacilli (GRGNB) isolated from patients, staff and their immediate environment were studied over a 6 month period . Of these 954 were isolated from clinical specimens obtained from patients and 540 from their immediate environment. A furthur 209 stains were isolated from the staff members who were responsible for the care of these patients. Pseudomonas aeruginosa; pyocin type 1 phage type F7 and .serotype 11 was the commonest isolate. It constituted 24,9% of all isolates in this study. This organism was distributed in all the wards investigated and was isolated throughout the 6 month study period. This strain, therefore, appears to be part of the "resident'' flora of King Edward Vlll Hospital for it was found on patients, staff and their immediate environment. Among the Enterobacteriaceae, Klebsiella pneumoniae was the commonest isolate and made up 13,6 % of all isolates. All the isolates obtained in this study were resistant to five of more antibiotics tested (gentamicin, tobramycin, kanamycin, streptomycin, carberricillin, polymyxin B amikacin and sisomicin). Of 310 staff members screened 25,2% harboured GRGNB on their hands. Among patients the commonest source of GRGNB was stool which yielded 141 (14,8 %) of the clinical isolates. Of the environmental sources studied, sinks harboured 87 (14%) GRGNB. The isolates from the environment and staff members were identical to patient strains. The significance of these findings is discussed. / Thesis (Fellowship of the Society of Medical Laboratory Technologists of South Africa)-University of Natal, Durban, 1985.