DNA from a temperate phage rho 11 and chromosomal DNA of Bacillus subtilis 168 were digested with endonuclease EcoRI and then ligated with T4 polynucleotide ligase. The ligated DNA fragments were used to transform a lysogenic strain, B. subtilis spoA12 lys21 hisA1 leuA8 p11, and Lys+, His+ or Leu+ transformants were selected. The cells of each type were then mixed, grown and treated with mitomycin C; the induced phages were tested for abilities abilities to form plaques and to tranduce the auxotrophic marker. Various types of plaque-forming or defective phages which transduce hisA or lys marker at considerably high frequencies were thus obtained.