In Petunia four alleles of the gene prxB could be identified by starch gel electrophoresis. Investigation of PRXb allozyme balance during development of tissue and plant of prxB heterozygotes showed differential temporal expression of the four mobility alleles. Analysis of developmental allozyme balance of prxB2/B3 heterozygotes from F1 and F2 progenies indicated the presence of a trans-acting regulatory gene termed Rp1. Segregation for the gene Rp1 indicated monogenic inheritance. Independent segregation of the genes Rp1 and prxB was found. When homozygous and recessive, the gene Rp1 speeds up the start of expression in enzyme activity of the prxB2 allele. The alleles prxB1 and prxB4 are expressed later in development than the alleles prxB2 and prxB3, no matter the Rp1 genotype. The final expression level of the prxB alleles is different for all four alleles, irrespective of the Rp1 genotype.