As reported elsewhere, an enterotoxigenic strain of Escherichia coli serotype O25:K98:NM was epidemiologically incriminated as the etiological agent in a shipboard outbreak of diarrheal illness. This enterotoxigenic E. coli strain and possibly other enteric isolates were found to produce heat-labile toxin and not heat-stable toxin. Since previous genetic analyses of enterotoxigenic E. coli strains producing heat-labile and heat-stable toxins have shown a plasmid location for both toxin determinants and since in this outbreak more than one bacterial strain appeared to produce only heat-labile toxin, the possibility of an extrachromosomal heat-labile toxin determinant was investigated. Results of endonuclease cleavage and hybridization experiments, as well as apparent heat-labile toxin phenotypic instability, strongly suggest a plasmid mediation of toxin production. Additionally, the stability of this heat-labile toxin production was evaluated after several traditional methods of bacterial cell preservation.