Conjugation-deficient mutants (Con-) of Escherichia coli K-12 have been isolated by a variety of indirect selective techniques. Mutants with mutations conferring ampicillin resistance, fosfomycin resistance, an alanine requirement, and a failure of ferment a number of carbohydrates were selected because the impaired functions occur in association with cell wall and cell membrane defects. The integrity of these catalytic or structural elements is postulated to have a role in conjugation. The mutants could be divided into at least six general categories corresponding to their defectiveness in the following postulated recipient cell functions: (i) specific-union formation, (ii) effective-union formation, (iii) deoxyribonucleic acid transfer, (iv) plasmid establishment, (v) plasmid maintenance, and (vi) recombination. The availability of these mutants should contribute to the description of the molecular events involved in each of these conjugation steps and the elucidation of these genetic control over the inheritance of conjugationally transferred deoxyribonucleic acid.