AbstractHybrids between R. parviflorus (diploid) on the one hand and tetraploid blackberry or diploid and tetraploid raspberry on the other were nearly as thornless as R. parviflorus. Degree of thorniness segregated as a continuous character in raspberry backcross generations. Diploid F1 hybrids with raspberry were almost completely infertile. Meiosis in PMCs and subsequent tetrad formation were normal but little stainable pollen was produced, although pollen fertility was adequate to produce a backcross to raspberry. Tetraploid F1 hybrids with raspberry were fully fertile. The F1 hybrids with blackberry were infertile as females and their flowers were devoid of functional stamens.