In order to investigate the adaptation of wheat to hot tropical environments, the phasic development and spike size of a wide range of wheat genotypes was studied during two winter seasons at four irrigated tropical sites in Mexico of similar latitude (18–20°N) but varying altitude (60–2640 m). The responsiveness of genotypes to photoperiod and vernalization was quantified in a growth cabinet experiment. The development of most genotypes was much faster and leaf and spikelet numbers in the field were reduced at the hotter (= lower) compared to the cooler sites. However, genotypes with marked photoperiodic sensitivity or responsiveness to vernalization (especially the latter) flowered almost as late at the lowest hottest site (mean minimum temperatures > 14°C) as at cooler sites. Delay due to lack of cold in the vernalization-responsive cultivars, confirmed by testing seed-vernalized material in the field, was effected up to initiation of the terminal spikelet and resulted in more leaves, spikelets and tillers than other cultivars at the hotter sites. Delay by photoperiodic sensitivity extended beyond terminal spikelet initiation. Duration of grain growth was uniformly shorter in all cultivars under the hotter conditions, irrespective of daylength or vernalization sensitivities. It is concluded that while photoperiod and vernalization sensitivity delayed the rate of development up until flowering, neither mechanism is likely to give improved adaptation to warm tropical conditions because the delay was not matched by increases in the number of grains per spikelet or per spike.