Of 37 Yersinia isolates from various aquatic environments, seven were I: enterocolitica and 30 I: intermedia. These isolates were biotyped, serotyped and tested for their susceptibility to 20 antibiotics. All I: enterocolitica isolates were of biovar 1; those of I: intermedia were distributed amongst four biovars (1, 2, 4 and 6). On the basis of combined biotyping and serotyping results, Y. enterocolitica isolates were distributed in five and X intermedia in 17 groups. With the exception of one Y. enterocolitica isolate which was resistant to tetracycline and streptomycin, the isolates were sensitive to the non-P-lactam antibiotics. In contrast, various patterns of p-lactam insensitivity were detected, including ampicillin and ticarcillin (35 isolates), cephalothin (33 isolates), carbenicillin (32 isolates), amoxycillin/clavulanate (23 isolates) and cefoxitin (22 isolates). No correlation between biotype or serotype and the susceptibility pattern of the isolates was apparent. Both inducible cephalosporinase activity against third-generation cephalosporins and inhibition of resistance to penicillins were detected in all I: enterocolitica and Y. intermedia isolates by double-disk tests.