Children in day care centers are frequently colonized with trimethoprim-resistant Escherichia coli.The frequency oftransmission to family members was studied. Colonization with trimethoprim- resistant E. coli was detected in 13 (57%) of 23 day care center children. Among the 50 (98%) of 51 household members tested, colonization was detected in 13 (26%), representing 12 (52%) of 23 households tested. Households of colonized center children had significantly more colonized members than did households of noncolonized children (odds ratio, 13.3; 95% confidence interval, 1.3–172.6; P = .01). There was a trend toward higher prevalence of colonization in mothers (35%) and siblings (30%) than in fathers (12%). The same plasmid profiles of trimethoprim-resistant E. coli from center children were found among 8 (67%) of 12 colonized households. There was no association between antibiotic use and fecal colonization with trimethoprimresistant E. coli.Thus, transmission of trimethoprim-resistant E. coli among children within day care centers and from center children to household members is common.