Two highly fertile and self-compatible dihaploids (2ns=2x24) from Solanum tuberosum L. (2n 4x 48) were investigated to elucidate the genetic basis of their self-compatibility. To this end the two dihaploids were selfed and reciprocally intercrossed and the resulting I1 and F1 plants tested for self-compatibility. Reciprocal backcrosses of I1-plants and F1-plants were made. Complete diallels both within self-compatible and within self-incompatible F1-plants were carried out as well as reciprocal matings between self-compatible and self-incompatible F1-plants. From the wealth of data it could be concluded, that the dihaploids have two intact S-alleles, one being common to both. Six hypotheses were tested for explaining self-compatibility in these particular dihaploids. All but one had to be discarded. It is concluded that the self-compatibility most likely is brought about by the presence of an S-bearing translocation, which is not linked to the S-locus. The ratio sc :si in the F1's point either to certative disadvantage of translocation-bearing pollen or to lethality of translocation homozygotes. The importance of this self-compatibility mechanism for genetic and breeding research in potato is discussed.