The effects of constant photoperiods (8, 9, 10, normal and 24 hr) and of transfer to another photoperiod at floral initiation (from 24 hr to 10 and vice versa) on rate of development and spikelet number per ear were studied in eight wheat cultivars grown at 20°C. The objective was to know what factors related to photoperiod control spikelet number. The lengths of the vegetative, spikelet and ear elongation phases, the numbers of spikelets and leaves, the numbers of phytomers and lengths of the shoot apices at floral initiation increased, but the rate of spikelet initiation decreased, as the photoperiod decreased from 24 to 8 hr. Responses to varying photoperiod for all these parameters were similar in the different cultivars but the sizes of the responses differed. Within a given cultivar, an increase in spikelet number was always associated with longer durations of the vegetative and spikelet phases and longer apices at floral initiation. The results of the transfer treatments suggest that spikelet number is not fully determined by the time of floral initiation, but can be altered significantly by manipulating the environment during the spikelet phase. It was concluded that the main factors determining spikelet number are rate and duration of spikelet initiation.