Number of days to flower (DTF) of 78 bean (Phaseolus vulgaris L.) genotypes was measured in tropical fields at various elevations. The associated 18 mean temperatures varied between 12 and 28C. Daylength was natural 12 or 13 hours of sunlight with or without incandescent light for a total of 18 hours. A statistical analysis with additive main effects and multiplicative interaction effects (AMMI) quantified the effects on the deviation from the DTF grand mean caused by each genotype, plus those caused by each daylength and by each temperature. The more photoperiod-sensitive the genotype (factor 1), the more a longer daylength (factor 2) increased DTF and the more a higher temperature (factor 3) synergistically increased DTF. These three factors interacted to delay the node to flower. An additional control over DTF occurred as the same higher temperature (factor 3) reduced the days required to develop a node (factor 4). Thus, a higher temperature tended to decrease DTF by enhancing the rate of vegetative development, at the same time that it tended to increase DTF by enhancing the photoperiod gene activity. This four-factor interaction resulted in a U-shaped curve of DTF in response to temperature. The smallest DTF on the U-shaped response was interpreted as occurring when the simultaneous effects of temperature toward earlier and later DTF exactly cancelled. At all temperatures below this optimum for flowering, a change of temperature changed DTF predominantly by altering the days required to develop a node. At all temperatures above the optimum, a change of temperature changed DTF predominantly by altering the photoperiod-gene-caused delay of the node to flower. The optimum temperature for flowering was lowered by higher sensitivity of the genotype to photoperiod and also by longer daylength.