Frost is one of the principal limiting factors for agricultural production in the high Andean region. One of the most important grain crops in that region, quinoa (Chenopodium quinoa Willd.), is generally less affected by frost than most other crop species, but little is known about its specific mechanisms for frost resistance. This study was undertaken to help understand quinoa's response to various intensities and durations of frost under different levels of relative humidity (RH). The effect of frost on seed yield and plant death rate was studied, and content of soluble sugars, proteins, and free proline, was analyzed, in order to develop criteria for the selection of cultivars with improved resistance to frost. On the basis of greenhouse and phytotron experiments, it was concluded that at the two-leaf stage, cultivars from the altiplano of Peru, 3800 m above sea level, tolerated -8°C for 4 h, whereas a cultivar from the Andean valleys tolerated the same temperature for only 2 h. At -4°C, plant death rate increased from 25% at high relative humidity to 56% at low RH After a frost treatment of -4°C applied at the two-leaf stage, final seed yield was reduced by 9% compared to control plants not exposed to frost. For the same treatment applied at the 12-leaf and flowering stages, yield reductions were 51 and 66%, respectively, indicating that frost for 2 h or more during anthesis caused significant damage to the plants. In general, an increased level of soluble sugars implied a greater tolerance to frost, resulting in higher yields. © 2004 Elsevier B.V. All rights reserved.