A second transducing bacteriophage, designated SP-15, was isolated from the same soil sample culture filtrate that supplied the Bacillus sub tilis transducing phage, SP-10, reported earlier from this laboratory. SP-10 and SP-15 differ serologically and in several other respects, but share the ability to propagate on B. subtilis W-23-Sr streptomycin-resistant and Bacillus licheniformis ATCC 9945A, and to mediate general transduction in either species when propagated homologously. Attempts to transduce between the species have failed. Sp-10 forms plaques readily on both W-23-Sr and 9945A SP-15 forms minute plaques on W-23-Sr and has shown no evidence of any lytic activity on 9945A. Maximum recoveries of prototrophic colonies from mixtures of SP-10 with auxotrophs of either W-23-Sr or 9945A were obtained only when excess phage was neutralized by post-transduction treatment with specific phage antiserum. Such treatment was not neces sary for maximum recovery of transductants effected by SP-15. SP-15 propagated on W-23-Sr did not transduce B. subtilis 168 indole, but transduced B. licheniformis more efficiently than SP-10. Neither phage transduced B. licheniformis as efficiently as it transduced B. subtilis..