Eighty-four transfer-deficient mutants of Flac have been isolated; 27 of these bear amber mutations and 1 mutant is temperature-sensitive. All the mutants transfer between 10(-2) and <10(-5)% as well as wild-type Flac, all are curable by acridine orange treatment, and all are resistant to the female-specific phage phi(II). Some of the mutants are partially sensitive to female-specific phage tau. Sixty-three of the mutants are resistant to the male-specific phages f1, f2, and Qbeta; 15 are resistant only to f2; and 6 are sensitive to all three male-specific phages. Most of the mutants are still poor recipients in conjugation, but four of the mutants resistant to f1, f2, and Qbeta have become good recipients. Those mutants resistant to all three male-specific phages do not seem to make F-pili.