Type 1 pili of Escherichia coli are the prototype of the somatic class of pili found on many strains of bacteria. As a first step in the genetic analysis of type 1 piliation, an extensive series of nonpiliated derivatives of E. coli K-12 strain AW405, was characterized to produce attached or free pili when examined in the antiserum or appeared to produce attached or free pili when examined in the electron microscope. The derivatives fell into two classes; phase variants and mutants. Phase variants that formed colonies of two distinctive types, one associated with a predominantly piliated (P+), and the other associated with a nonpiliated (P-) phase, were obtained. Each phase could give rise to the other at a relatively high rate, which was greater in the P- to P+ direction during culture in unshaken liquid medium. In addition, 77 Pil- mutants were selected on the basis of a subtle difference in colonial morphology. The mutants reverted, if at all, at a much lower rate than that of the P- to P+ change. The stability of Pil- derivatives grown in unshaken liquid medium was used as a criterion for distinguishing between phase variants and mutants, Phase variation also effected colonial morphology and chemotactic swarming. These properties did not directly depend upon piliation since Pil- mutants were only slightly altered in colonial form and unaltered in chemotactic swarming. Piliation of Pil+ bacteria was quantitatively affected by growth conditions.