Sixty-seven isolates of citrate-positive variants of Escherichia coli were isolated from human, domestic animal, feral bird and environmental sources. With the exception of citrate utilization, all isolates were identified as typical E. coli by their biochemical reactions. The transmission of the ability to utilize citrate on Simmons' citrate agar was demonstrated in 53 (79·1%) out of the 67 citratepositive E. coli variants obtained from various sources. Drug resistance determinants and citrate utilizing character were co-transmitted into E. coli K-12 by conjugation among citrate-positive E. coli isolates carrying R plasmids except for that isolated from horses. The other characters (haemolysin or colicin production, raffinose or sucrose fermentation) were not transmitted together with the citrate utilizing character. These facts suggested that the structural gene responsible for citrate utilizing ability in citrate-positive variants of E. coli was located on a conjugative plasmid.