1. Male and female Y-chromosome hyperploid plants were obtained and morphologically and cytologically investigated. 2. Conjugation of sex chromosomes was investigated in ♂ plants with 1, 2 and 3 additional Y chromosomes. Conjugation between sex chromosomes was end-to-end. Association between Y-chromosomes was partly based on conjugation between homologous segments and partly due to heterochromatin fusion. 3. Transmission of the additional Y chromosome to the progeny was studied. In all cases transmission of the extra Y chromosome was very low. The reasons for this phenomenon are probably nonrandom fertilization, nonrandom degeneration of megaspore, and reduced seed setting and seed fertility in plants with additional Y chromosomes. 4. The phenotypic effect of Y chromosome hyperploidy was investigated. Plants with extra Y chromosomes are shorter and die earlier. The fertility of such plants is low. Thus, although Y chromosomes are heterochromatic and nonactive (except in the premeiotic interphase of microsporogenesis) they have a marked influence on the appearance of Y-chromosome hyperploid plants.