The serotyping of the Pseudomonas aeruginosa strains isolated from various clinical materials is of great epidemiological importance, and distribution of the serotypes of P. aeruginosa has been reported throughout the world during the past 20 years (1-15). We have serotyped 668 P. aeruginosa strains isolated in Israel from diverse clinical specimens. This is the first report dealing with the distribution of the serotypes of P. aeruginosa in Israel.The 668 P. aeruginosa strains were isolated from various clinical specimens such as urine, pus from wounds or ears, faeces, sputum or throat swab from respiratory tract or blood, in Rothschild University Hospital, Nahariya Government Hospital and Poriah Government Hospital, Israel, during the years 1972-1976. During this period there was no epidemic incidence of P. aeruginosa in any of these hospitals. The strains with the following characteristics were identified as P. aeruginosa: Gramnegative motile rods which oxidized glucose, but not maltose, reduced nitrate to nitrite, gave positive cytochrome-oxidase reaction ; they did not decarboxylyse lysine in lysine-iron agar, but dehydrolysed arginine in bacto arginine assay medium;4 gelatin was liquefied; indole was not formed; they did not produce H25 in TSI agar; gluconate was oxidized; aesculin was not hydrolysed.The antisera used were prepared as follows: the P. aeruginosa strains sent from Lanyi, representing his serotypes 01, 03a, 3b, 03a, 3d, 04a, 4b, 04a, 4d, 05a, 0b, 5c, 06, and 07a, 7b, were cultured on trypticase soya agar (Difco) containing 5% horse blood and incubated at 32 C for 18 hr. The culture was harvested, washed once with saline and then boiled for 2 hr. The boiled suspension was injected intravenously into rabbits, repeatedly with increasing dosages. Ten days after the last injection the rabbits were bled and the serum was stored at -20 C with 0.5% phenol.The slide agglutination test, using living cultures grown on nutrient agar at 233