Escherichia coli strains that cause dysentery-like disease, parenteral infection, and infantile diarrhea form specific groups based on mobility of O and K antigens in immunoelectrophoresis. Members from each of these groups were assayed for gross nucleotide sequence relatedness. The method used was interspecific deoxyribonucleic acid (DNA) reassociation reactions carried out free in solution. Reassociated DNA was separated from unreacted DNA by passage through hydroxyapatite. DNA relatedness between these groups was approximately 80%. The groups containing those strains causing parenteral infection and those responsible for dysentery-like disease showed preferentially high intragroup DNA relatedness. The group containing strains responsible for infantile diarrhea did not show preferentially high intragroup DNA relatedness with the reference strain employed. These strains, however, did exhibit preferentially high DNA relatedness to a second reference strain.