At three loci four phenotypically different mutations were induced by x rays which affect the photoperiodic response of the carrier. The standard wild type develops visible flower primordia in 10 to 12 days after germination. The mutants have various flowering response and may need a period up to 4 to 6 times as long for the completion of the same developmental stage. In spite of the large differences in the time required for the onset of fiowering, the life cycle of the latest mutant is only 2 to 3 times that of the wild type. The slower development is accompanied by very vigorous growth; dry matter production may increase over 20 times and seed production may rise tenfold. These hereditary changes do not involve impaired genetic transmission, altered crossing-over value or visible chromosomal alterations. In three different competitive populations (mutant x wild) under long-day conditions, in the greenhouse, during a 60-week period (about ten generations of the wild type), the mutant allele increased from the initial 0.500 frequency to 0.869, 0.995, and 0.997, respectively. The roughly estimated selective advantage of the mutants under the experimental conditions varies from 0.3 to over 1.0. These are extremely high values. Since the progressmore » of selection is rapid in self-fertilizing plant community, if vigorous late mutants occur at a fair rate, their rapid increase may be expected. In nature late ecotypes are prevalent. The possible evolution of the photoperiodic response in the species is discussed. The suggestion is made that the different degree of dominance of the late ecotypes, based on modifier genes, indicates the evolution of dominance of the character concerned and the late flowering forms in nature represent the emergence of the new wild type of the species. (auth)« less