The colonizing and adhesive attributes of enterotoxigenic acapsular and/or nonpiliated mutants from K88-negative enteropathogenic Escherichia coli strains were compared with their capsulated and piliated parents (parents were piliated when grown in vitro and in vivo). Acapsular, nonpiliated mutants from three different colonizing strains of enteropathogenic E. coli lost their ability to colonize the ileum of newborn pigs. Acapsular, piliated and capsular, nonpiliated mutants were derived from one of the parental strains (987), and both mutants lacked the ability to colonize the ileum of pigs. The only mutants available from a fourth strain (431) were acapsular and piliated, and they colonized as well as their parents. These data indicate that both capsule and pili are involved in colonization by strain 987. In contrast, capsule is not required for colonization by strain 431, but pili may be.