Previous workers have shown that intergeneric crosses between Salmonella typhimurium and Escherichia coli produce a high proportion of merodiploid recombinants among the viable progeny. We have examined the unequal cross-over event that was responsible for a number of intergeneric merodiploids. The merodiploids that we studied were all heterozygous for the metB-argH interval and were the products of intergeneric conjugal crosses. We found that when the S. typhimurium donor had its transfer origin closely linked to metB and argH, all recombinants examined were merodiploid, and they generally arose as F-prime factors. Many of these F-prime factors had been created by recombination between flanking rrn genes in the donor. When the S. typhimurium Hfr transfer origin was more distant from the selected markers, quite different results were obtained. Depending on the donor, 19-47% of the recombinants that acquired the donor argH+ or metB+ genes were merodiploid for these loci, but none of the recombinants were F-prime. A majority of the merodiploids had a novel (nonparental) rrn gene, indicating that unequal recombination between nonidentical rrn genes was a prevalent mechanism for establishing the merodiploidy. Both tandem and nontandem duplications were found. Some of the merodiploids duplicated E. coli genes in addition to acquiring S. typhimurium genes. Some merodiploids contained the oriC region from each parent. Of a total of 118 intergeneric merodiploids characterized from all donors, 48 different genotypes were observed, and 38 of the 48 had one or more nonparental rrn operons.