Thirty-two amaranth accessions from the world's collection were hybridized to produce eighty-seven Fls which were evaluated for heterosis and combining ability in three factorial mating designs. On average, interspecific matings of Amaranthus cruentus L. and A. hypochondriacus L. produced significant (P = 0.01) biomass increases over their parents. With A. cruentus African vegetable accessions as females, male parents of both species showed significant (P = 0.05) general combining ability (GCA). Midparent heterosis for biomass ranged from -13% to 88%. Late flowering of interspecific matings strongly influenced the larger biomass yields. Intraspecific matings of Asian A. hypochondriacus accessions exhibited highly significant (P = 0.01) accession heterosis for biomass and GCA and SCA effects. Intraspecific hybrids among A. cruentus grain and vegetable types showed significant (P = 0.05) GCA effects for the grain types when used as male parents but average heterosis was zero. Interspecific hybridization may be a promising way to increase biomass productivity of Amaranthus;Efficient techniques to screen genotypes for superior biomass production are highly desired in breeding programs for forage, vegetable, and bioenergy. Basal stem diameter gave a good estimation of vegetative biomass in three factorial matings of grain amaranths. Rank correlations of these two traits were 0.97, 0.84, and 0.89 for 16, 56, and 16 crosses, respectively. In all matings, GCA for stem diameter was evident for grain type males;In a study of genetic variation for selected traits in Amaranthus cruentus L., eight and two accessions were harvested at two-week intervals in 1985 and 1986, respectively. In both years, between 40 and 100 days after planting, the accessions displayed a linear growth phase for above-ground dry weight. Grain yield was influenced by date of flowering and most grain accessions showed maximum grain yields at 114 days after planting. When compared to random accessions, improved accessions had significantly (P = 0.05) higher total leaf area, and above-ground and total dry weight. PI 482051, a Zimbabwean vegetable accession, had more than twice the below-ground biomass when compared to other accessions.