Abstract A Petunia hybrida mutant that self-pollinates was identified in an M1 generation of plants mutagenized with ethyl methanesulfonate. A change in floral morphology of the self-pollinating (SP) mutant resulted in flowers with the stigma and anthers in direct contact. Other phenotypic changes in the SP line, which are maintained in M2 and M3 progeny, included alterations to leaf, fruit, and seed morphology. Flow cytometry and cytogenetic analysis found that the SP mutant had twice the nuclear DNA content and chromosome number of the parental line ‘Mitchell Diploid’. Polyploidy may be responsible for some, but not all, of the phenotypic changes in the SP mutant. Both self-pollination and polyploidy could be advantageous for the genetic screening of petunia for induced mutations in candidate genes.