Salmonella typhimuriummutants sensitive to coliphage P1 are resistant to salmonella phage P22 and lose their P1-sensitivity upon reversion to P22-sensitivity. A tryptophan-requiringSalmonella typhosa–Escherichia colihybrid, which has the unique ability to serve as recipient in transduction mediated by both P22 and P1, was used to determine if P22 and P1 adsorb at the same oroverlapping sites: (i) The adsorption of each of P1 and P22 is similar when added individually or together to the hybrid at a saturating multiplicity of infection (moi), (ii) P1 grown ontrp+E.coliyields the maximum frequency of Trp+abortive transductions at an moi of 6 with thetrphybrid recipient; the presence of increasing numbers of P22 grown ontrp S.typhimriumdoes not decrease the number of Trp+transduction from P1. (iii) A mixture of P1 (grown ontrp+E.coli) and P22 (grown ontrp+S.typhimurium) yields more abortive transductions than does P1 alone. Thus phages P1 and P22 adsorb to the hybrid cells on different sites.