The activities of cefsulodin and gentamicin against Pseudomonas aeruginosa isolated from clinical specimens from five hospitals in different geographical areas of Japan, from 1980 to 1983, were compared in vitro. The incidence of resistant strains was higher for gentamicin. In 1982 the sensitivity to both drugs decreased from 1980 and 1981 levels, largely because of the isolation of numbers of cefsulodin-gentamicin cross-resistant bacteria from three of the five hospitals. In 1983, the incidence of resistant strains was similar to that in 1982. This linked cefsulodin-gentamicin resistance may have been selected by the use of the cephalosporins, cefmenoxime, cefoperazone, cefotaxime, ceftizoxime and latamoxef, which had been prescribed extensively in the hospitals where cross-resistance was encountered. Although cefsulodin-resistant strains of P. aeruginosa have increased since 1982, the in-vitro activity of cefsulodin in 1983 remained greater than that of these third-generation cephalosporins.