Transduction of plasmid pC194 and bacteriophage phi 11de varied inversely with the multiplicity of infection. As the multiplicity of infection decreased from 10(-1) to 10(-5) PFU/CFU, the transduction frequency of pC194 increased 10(4)-fold; the transduction frequency of phi 11de increased 300-fold with a 100-fold decrease in multiplicity of infection. Physical and genetic analysis of the transduced DNA showed that pC194 resided in the phage particle as a random, circularly permuted linear concatemer. In DNA prepared from phage that cotransduced pC194 and phi 11de, pC194 resided in the transducing phage primarily as a linear multimer of 15.8 kilobases, or about 5.4 pC194 monomers. The pC194 multimer was randomly inserted into the phi 11 genome.