In the last decade in Ontario a new strain of E. coli (ETEC) of the serogroup O149 has been causing severe diarrhea in piglets after weaning. This new strain contains pTENT2 which encodes for tetracycline resistance and virulence factors that were absent from previous O149 ETEC strains. This suggests that the increased virulence of the new ETEC is caused by the pTETN2 plasmid and that the use of tetracycline may select for this strain. The objectives of this project was to develop methods to identify and detect this strain, to generate mutants without the new pTENT2 plasmid, and to compare the behavior of the ETEC strain with and without pTENT2 with and without tetracycline in the feed in an animal model for postweaning diarrhea. E. coli O149 strains with and without the pTENT2 plasmid were generated. They were used to infect weaned piglets fed with and without tetracycline. The severity of the treatments was assessed in the piglets.No statistical difference was detected between groups that received the modified ETEC strains. Some variations were found, but studies with larger numbers need to be carried out to confirm the significance.