In vivo and in vitro recombination methods were used to construct the recombinant plasmid pTG801, in which the F-plasmid DNA transfer (tra) genes required for the formation of functional F pili were placed under the lac transcriptional control sequences of pUC19. The 20 kilobases of cloned F DNA includes genes traA through the 5'-terminal part of traG; the plasmid lacks the positive regulatory gene traJ and all of the known tra genes required for the DNA transfer stage of conjugation. pTG801 transformants were sensitive to the donor-specific bacteriophages Q beta and f1, as measured by the formation of infectious centers. They were relatively insensitive to bacteriophage R17, as expected from the absence of traD. In the presence of a lacIq allele, sensitivity of pTG801 transformants to f1 and Q beta depended on the concentration of inducer (isopropyl-beta-D-thiogalactopyranoside [IPTG]). Viewed by electron microscopy, pTG801 transformants elaborated 7- to 10-nm-diameter filaments that could be laterally decorated with RNA bacteriophage particles, consistent with the formation of F pili. In stationary-phase cultures, these filaments formed massive aggregates and could be seen to adhere lengthwise to the cell surface; few pili accumulated in the medium as single filaments.