The F'lac+ episome of Escherichia coli origin was transferred by conjugation with frequencies of 10(-7) to 10(-5) from Erwinia amylovora to 14 out of 15 Salmonella typhimurium trp female parents. The chromosomal trp+ genes were transferred with frequencies of 10(-7) to 10(-6) only to one trpB and 2 trpD female parents, which have a point mutation in the 2nd and fourth structural genes, respectively, of the tryptophan operon. The transferred male trp+ genes became integrated at the selected sites of the S. tryphimurium chromosome. The resulting Trp+ hybrids were phenotypically stable, lacked a cryptic trp allele selected against in the female parent, had high genetic homology values in the tryptophan region, and showed biochemical reactions and pathogenicity typical of S. typhimurium.