The lys gene of Bacillus subtilis was inserted into prophage phi 105. The recombinant phage (phi 105dlys) contained DNA which was about 2 MDal smaller than the wild-type phage DNA, and the phage particles had no tails. The phage did not plaque but, when provided with tails in vitro, it transduced both lys-1 and lys-3 strains of B. subtilis to Lys$. The lys$ gene was located on a 2.5 MDal EcoRI restriction fragment. Subsequently this phage was phi 105 105dspoIIIB, was also defective, i.e. without tails. The DNA was 1.5 MDal smaller than the wild-type phage DNA and the spoIIIB2$ gene was located on a 3 MDal EcoRI fragment. When provided with tails in vitro, phage phi 105dspoIIIB transduced cells of a spoIIIB2 recipient to Spo$. In these transductants the spoIIIB2 mutation was complemented, and the cells sporulated normally.