The in vitro susceptibilities of 145 Photobacterium dantselae subsp. piscicida strains, isolated from different geographical regions, against 15 major fish farming antimicrobial agents were assessed using a disc diffusion method. Ninety-nine percent of all isolates were sensitive to florfenicol and trimethoprime-sulfamethoxazole, 93% of the strains were resistant to erythromycin. Based on the antimicrobial susceptibility, the P. damselae subp. piscicida isolates could be distinguished from each other. While all American and European isolates were sensitive to kanamycin, 49% of the Japanese isolates were resistant to it. Moreover the results clearly indicated increasing resistance to a variety of antibiotics, particular in Japan, but also in some Mediterranean countries. (C) 2001 Elsevier Science B.V. All rights reserved.