Quinoa cultivars currently grown in North America and Europe require removal of bitter-tasting saponins from the grain prior to human consumption. This need for postharvest processing is a barrier to expanding production of the crop outside its Andean area of origin. Grain saponin content in quinoa shows continuous variation and is considered to be a quantitative trait. However, segregation for the presence or absence of grain saponin in F2 generations derived from crosses between high- and low-saponin parents indicates a major gene effect, with plants homozygous for a recessive allele spl having no detectable grain saponin. Variation in saponin levels among F2 plants with detectable grain saponin was consistent with polygenic inheritance. It appears that grain saponin level in quinoa is both qualitatively and quantitatively controlled, with saponin production requiring at least one dominant allele at the Sp locus and the amount of grain saponin being determined by an unknown number of additional quantitative loci. Introgression of sp1 into day-neutral lines will facilitate the development of short-season "sweet" quinoa cultivars which do not require postharvest processing to remove grain saponin.