Pooled faecal samples of weanling pigs were collected in 1992 on 32 purposively selected farms in Ontario. 4356 Escherichia coli isolates were tested for resistance to seven antimicrobials: ampicillin, carbadox, gentamicin, nitrofurantoin, spectinomycin, sulfisoxazole, and tetracycline using a hydrophobic-grid membrane filter method. Logistic regression analysis was used to model associations between antimicrobial resistance and drug usage patterns among faecal E.coli of weanling pigs at the farm level. The in-feed addition of penicillin, carbadox, sulphonamide or tetracycline to weanling pig rations ("starter rations") was associated with increased risk of resistance to ampicillin, carbadox, sulfisoxazole and tetracycline, respectively. Management and housing factors were also significantly associated with the risk of resistance but without any consistent pattern. These results indicate that a prolonged antimicrobial exposure to groups of pigs in-feed is more consistently associated with increased risk of resistance than individual animal treatment.