Gametophytic self-incompatibility in flowering plants can be explained in terms of recent concepts of gene action. The S alleles may be assumed to be regulators which produce monomers of a dimer repressor controlling a high rate of growth operon in the pollen tube. Similarity between monomers in the style and pollen tube produce a functional dimer repressor which inhibits this operon. A low velocity growth operon yielding pollen tube growth typical of incompatible pollen tubes would function when the high velocity operon is inactive.