Bacterial hybrids were produced to contain genetic material of Salmonella typhimurium, Escherichia coli, S. montevideo and S. abony origins. Analyses by transduction provide evidence that 6% of the original typhimurium genome has been replaced in the production of these hybrids. Although a number of biosynthetic pathways are affected by this gene substitution, the growth rate of these hybrids in minimal medium is unchanged. Supporting evidence for the close relatedness between S. typhimurium and the other three species is not observed in recombination studies. Available results favour the concept that differences in base sequences are responsible for the low frequency of recombination obtained in heterologous crosses.