The hybrid varieties known under the collective vernacular name of Petunia (Stimoryne hybrida (Hook.) Wijsman) have been derived from crossing two ancestral species,, S. axillaris and S. integrifolia. Formerly the two species and their hybrid were included into the genus Petunia Juss., but recently they have been transferred to Stimoryne Rafin. (Wijsman & de Jong 1985). The difference between S. axillaris with large white flowers with a slender long flower tube, and S. integrifolia with smaller, purple flowers so wide as to obscure the border between limb and tube have been described (Wijsman 1982, 1983) and can be seen in fig. I. Because a considerable number of genes has been characterized in the hybrid (de Vlaming et al. 1984), it seemed of interest to investigate whether in addition to the genetic basis of the difference in flower colour (Wijsman 1983), the genetics of the difference in flower morphology between the two species could be understood. Two lines have been crossed, S2 (. S. axillaris ssp. axillaris) and S6 (S. integrifolia ssp. inflata). differing in several factors located in most linkage groups. Some of the genetic differences (see de Vlaming et al. 1984) can be determined visually, like FI (on chromosome II), Po (V), An2 (VI), others by chromatography like FI. The difference as to FI is blurred by the presence of modifiers in S6, which has relatively much flavonol. The genes prxB (I), prxA (III), and prxF (VII) concern electrophoretic differences in isoperoxidases. Line S6 was formerly heterozygous forprxC; unfortunately no F1 heterozygous forprxC could be found, and no other marker on chromosome IV segregated.