A study was made of the antigenic variation of K. coli isolated from one individual (individual A) over a period of 14 months. Of 650 E. coli isolated from individual A during this time. 85.3% were antigenically identical with one or another of 10 strains from individual A which were used to prepare 10 antiserums. 16% of 75 E. coli isolated from 3 other members of the family of individual A and only 1% of 100 E. coli from 100 unrelated individuals were antigenically identical with one or another of the 10 homologous strains. An antigenic type of E. coli seemed to dominate in the feces of individual A for a limited time, then disappear, giving way to another antigenic type. Antigenically identical but biochemically different strains were occasionally isolated from one fecal specimen.