Seed development was studied in crosses made between the diploid forms and autotetraploid mutants of two raspberry (Rubus idaeus L.) cultivars. Drupelet abortion did not appear to be caused by failure of fertilization; it occurred mainly between the fourth and eighth day after pollination, and manifested itself as simultaneous cessation of growth in all tissues. In healthy drupelets at eight days after pollination, growth appeared to be proceeding at similar rates in all the tissues except that the embryo seemed to be growing faster than the endosperrn in the most advanced crosses. Both diploid and tetraploid ovules appeared to be more stimulated by pollen from tetraploid plants, Embryo differentiation differed according to the direction of cross, especially in stunted embryos. Radicles tended to be abnormally large when diploid pollen was used and smaller than usual with haploid pollen.