Solanum tuberosum Group Tuberosum haploids (2n = 2x = 24) were inter-mated using bulk pollen. Nearly half of the resulting progeny were tetraploid, arising from bilateral sexual polyploidization. Subsequent yield trials indicated that the diploid progeny did not yield significantly more than the original haploids, but the tetraploid progeny did yield significantly more than their diploid sibs and the original haploids. Tetraploid cultivars and conventional tetraploid breeding families significantly out-yielded the haploid-derived tetraploids. These results and the theoretical considerations which are presented indicate that inter-mating haploidsper se does not increase heterozygosity. More importantly, these results indicate that without the introduction of new germplasm, haploidization followed by repolyploidization can only be expected to result in tetraploids with heterozygosity equal to or below that of their parents.