||Vernalization requirement and response to day length are the major factors controlling development in wheat (Triticum aesitvum L.). This study was to determine how they integrate in the field in Obregon, Sonora, Mexico to result in development phenotype. In addition to a natural day-length treatment, day length was shortened in a second treatment by covering the crop from 0.5 h before twilight until 1.0 h past sunrise, extended in a third treatment by illumination from 0.5 h before until 3.0 h after sunset for the entire season, and extended by the same amount in a fourth treatment beginning 47 days after planting. Development of one facultative, one winter, six spring cultivars, and ten F)-derived F5 lines from the spring-spring cross Pfau/Weaver was followed by dissection of plants every two days. In addition to the nonvernalized treatment, cultivars were also subjected to six weeks of vernalization in darkness at 6°C prior to planting in the field. Beginning two days after germination in the field, the spring, facultative, and winter cultivars Oasis, TAM 200, and CenturklVeery 5, respectively, were also moved to a cold chamber and subjected to 6°C and day lengths simulating those outdoors for three weeks before being returned to the field. After germination, all entries were insensitive to day length. During this phase, development was vegetative and no progress was made towards flowering. Without vernalization, this juvenile phase was 68 and 40 d long in winter and facultative cultivars, respectively, and ranged from between 20 and 33 d in spring genotypes. The subsequent day length sensitive phase was always associated with the shift from vegetative to reproductive development. All cultivars, when vernalized for six weeks and subsequently transplanted to the field on the same day when nonvernalized plants received their initial irrigation, shifted from vegetative to reproductive. development earlier than their nonvernalized counterparts. A vernalization treatment after normal germination in the field, either extended or did not effect the duration of the juvenile phase. After the juvenile phase, all entries developed more quickly towards flowering the longer the day. Whenever an entry had initiated the spike, spikelet, or floret prior to day 47, when the second extended day-length treatment was imposed, the development of the respective organ was not influenced. The later flowering of winter and facultative than that of spring wheats is due largely to their long juvenile phases. Vernalizing temperatures do not shorten the juvenile phase through a response to cold per se, but rather by allowing part of it to be transgressed during a time of minimal growth. After the juvenile phase, all genotypes move into the reproductive phase with a concomitant response to day length. They do not vary in being either sensitive or insensitive to day length, but rather in the rate of development towards flowering which any given length of day provokes. Response to day length determined the duration of spikelet initiation and how quickly each spikelet progressed towards flowering.