An operon fusion of the lac genes to those required for synthesis of type 1 fimbriae (pili) has been achieved in a K12 strain of Escherichia coli lysogenized by the bacteriophage mu d (Ap4, lac). Synthesis of beta-galactosidase, therefore, reflected pil gene transcription and was used as a probe of fimbrial regulation. Expression of the operon fusion was found to oscillate, demonstrating that phase variation between fimbriate and nonfimbriate states is under transcriptional control. The transition rates from fimbriate to nonfimbriate were 1.05 X 10(-3) per bacterium per generation and from nonfimbriate to fimbriate, 3.12 X 10(-3) per bacterium per generation.