ABSTRACT In Argentina there have been few evaluations of Andean pseudocereal plantings. This study explored the response of Amaranthus cruentus cv Mexicano to different plant densities under furrow irrigation in the lower valley of the river Negro, Patagonia, Argentina. The experimental design consisted of 3 blocks with randomized treatments (subplots), each one corresponding to different plant density. The treatments were sown in rows with spacing of 0.70 m (one row per ridge) and others with a spacing of 0.35 m (two rows per ridge). The plant densities evaluated were: 70,800 - 84,200 - 97,700 - 116,000 - 114,000 - 225,300 and 394,000 plants ha-1. Different biometric variables and their components were measured: plant height, number of leaves, biomass and economic yield. The results suggest that the optimum plant density was 116,000 plants ha-1 with a row spacing of 0.70 m. This density produces an adequate plant stand from which to harvest optimal biological and economical yields. The contributions of this study demonstrated the potential of the A. cruentus crop in the lower valley of the river Negro, representing the southernmost study of plant density made for this pseudocereal in the world.