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Under rainfed conditions in the semi-arid highlands of Mexico, time to first Bower and to maturity of some cultivars of common bean (Phaseolus vulgaris L.) is strongly reduced with late plantings. Consideration of rainfall patterns in the region suggested that this response might be an adaptation to growing seasons of variable length, where length is determined by time of onset of summer rains and by risk of water deficit or low temperatures at the end of the season. Analysis of weather patterns at two sites in the highlands suggested that when rains commenced in early June, there was a high probability of adequate precipitation during the season. When rains commenced in mid- to late-July, total precipitation was lower. For a third site, total precipitation showed no relation with date of onset of rains. Evaluations of 20 bean genotypes planted at five dates each in 2 yr at Pabellon, Aguascalientes, (20 degrees 11’ N latitude, elevation 1910 m) demonstrated large differences in plasticity of the pre-flowering and reproductive periods. Cultivars Garbancillo Zarco and Tlaxcala-475 were late to Bower and mature under early plantings, but were similar to other genotypes in late plantings. Other genotypes of highland origin showed little or no plasticity, suggesting that this trait is not universal among cultivars currently grown there. Furthermore, cultivars and breeding lines from other regions showed no plasticity. Marked phenological plasticity occurred only in genotypes from the highlands which were highly photoperiod sensitive, but several highly sensitive genotypes shelved no or little plasticity. Similar patterns were noted in a survey of 81 genotypes that included a wider range of highland germplasm.