The tamarillo,Cyphomandra betacea(Cav.) Sendt. (Solanaceae), is a minor fruit crop grown in New Zealand and in subtropical climates elsewhere around the world. There is little genetic variation in the cultigen, but characters of commercial importance have been observed in related wild species.Cyphomandra betaceawas crossed with nine otherCyphomandraspecies. Generally, fruit set was poor and no viable seed was set in the crosses attempted. The use of polyploid forms ofC. betaceain some species crosses did not enhance the success of interspecific hybridisation. Various stages of cross failure were identified, most of which were post-zygotic. The data would suggest that incongruity, rather than the S locus, is governing interspecific compatibility in this genus. Suggestions are made for the future genetic improvement of the tamarillo by interspecific hybridisation.