Thirty‐five genotypes of grain amaranth were evaluated for adaptability and yield performance at three locations [lowland rain‐fed areas at 300 meters above sea level (MSL); upland, 1000 MSL; and highland, > 1000 MSL] in Chiang Mai, Thailand, from 1982 to 1987. Results showed that there were marked differences in yield stability and adaptation of various types of grain amaranth. “African” and “Mexican” morphological groups had higher yield and greater adaptability than the other grain types; this was probably due to the higher crop growth rate, seed protein content (>17%), and drought resistance of these two groups. The “South American” group was least well adapted to the lower altitude, but it performed well under highland conditions. In upland areas, accessions 83S‐27 and R‐158 had the highest average grain yields (3338 and 2938 kg/ha, respectively). The average seed yield of 35 genotypes grown in the lowland and highland areas was lower (86% and 32%, respectively) than that grown in the upland. These results were probably affected by drought stress in the lowland rain‐fed areas and by cold temperatures at the highland locations.