A mutation found in the raspberry variety Mailing Jewel had bigger fruiting laterals, flowers and fruits than the parent stock. The results of breeding experiments were consistent with the hypothesis that the change was caused by mutation of a single gene from the homozygous to the heterozygous state, and that the gene concerned has functions whose effects are seen at each stage of development. These effects were to depress slightly the amount of vegetative, or first year's, growth and to increase the amount of reproductive, or second year's, growth. It is suggested that genes of similar function may account for differences which distinguish the raspberry from certain related species. The mutant gene is useful for plant breeding purposes. Its possible effects on the physiology of the plant are also discussed.