Although the overall incidence of ectopic gestation has been rising over the past 30 years, unilateral twin tubal gestation is still a relatively rare event. In the 93 cases reported to date, zygosity determined solely by subjective observations of the fetuses indicated monozygosity in more than 95%. We report a case of unilateral twin tubal gestation in which zygosity of the twins was determined by using DNA probes that detect restriction fragment length polymorphisms. We used six human DNA probes and M13 phage DNA to compare the genotypes of the twins. Three of the six human probes and the M13 probe showed differences, proving that these twins were dizygotic. We speculate that many of the unilateral ectopic twins who were thought to be monozygotic may actually have been dizygotic.