The name selfer has been given to a mutant strain of bacteria in which transduction with homologous phage gives rise to a significantly larger number of wild-type variants than occurs by spontaneous reversion in uninfected control bacteria. Experiments with 201 auxotrophic mutants of Salmonella typhimurium representing 22 gene loci carried in 12 transucing fragments showed that about 40% of these mutants are selfers. In similar experiments with 32 auxotrophs representing four histidine loci, one selfer has been detected. A statistical analysis has revealed that selfer mutations are not distributed at random either within or among the loci where they are known to occur. The frequency of wild- type variants in selfing is evidently connected in some way with the transduction process. The excess variants cannot be transductants, however, since they appear also in crosses between selfer recipients and donors that carry a deletion covering the selfer gene. It is proposed that they are revertants: that the presence of a transducing fragment in the proximity of a selfer gene, when the fragment synapses with the homologous region of the bacterial chromosome, stimulates the gene's mutability. (auth)