Great millet of 35 varieties from several tropical and subtropical countries were tested for response to photoperiod. Some were analysed and measured extensively with photoperiods ranging from 5 to 24 h in a 24-h day. The development of the growing point was observed in relation to duration of growth from sowing until flower initiation and anthesis. Leaf number, elongation and length of leaves, stem and panicle, dry weight of plant and panicle were noted. The influence of solar radiation, intensity of supplementary light and temperature were, where possible, recorded.Great millet was a short-day plant highly variable between varieties and types. A range of variations was found 'from quantitative to entirely qualitative responding plants'. In general photoperiodic sensitivity of types decreases with increasing distance from the equator. The optimum photoperiod was about 10 to 11 h in a 24-h day for 10-14 days (dependent on age). Flower initiation starts 4 to 5 weeks after sowing, anthesis 4 to 6 weeks later. A minimum age must be reached before flowering could be induced.