Most virulence genes of enterotoxigenic Escherichia coli (ETEC) are located on plasmids. The gene for heat-stable enterotoxin I (sta) is part of the transposon Tn1681, and the heat-stable enterotoxin II (stb) gene was described to be part of the transposon Tn4521. In the studies presented here, we describe the linkage of the sta and stb genes on an approximately 10-kb fragment designated as toxin-specific locus (TSL). The TSL has been isolated from the 120-kb virulence plasmid pTC of the porcine ETEC strain 2173 that produces F18 fimbriae. The nucleotide sequence of the TSL fragment was determined. Sequences in the flanking regions of the sta gene indicated the presence of Tn1681, but--unexpectedly--flanking sequences of the neighbouring stb gene were completely different from those of Tn4521. The 10-kb TSL is part of a 40-kb fragment that contains the replication origin of pTC. This 40-kb fragment was mobilised into plasmid pACYC177 and the nucleotide sequence of the bordering fragments was determined. The 40-kb fragment was flanked by IS10 elements at both ends, indicating the existence of a new 40-kb pathogenicity island (PAI) in strain 2173. Several F4(K88)+ ETEC and F18+ ETEC as well as F18+ E. coli strains producing enterotoxins and verotoxin-2 (ETEC/VTEC) from weaned pigs of different geographical origin were tested for the flanking regions by PCR to see if they belong to the "Tn4521-like" or the "pTC-like" stb type. It turned out that the Tn4521-like stb-type was characteristic of F4(K88)+ ETEC, while the pTC-like stb type was present in most F18+ ETEC and F18+ ETEC/VTEC, although polymorphism was observed both in the K88 and F18 groups. These results suggest the existence and worldwide spread of a new plasmid-encoded pathogenicity island in porcine post weaning ETEC and ETEC/VTEC strains producing F18 fimbriae.