Two cultivars of Echinochloa utilis, Ohwi et Yabuno, and one E.frumentacea (Roxb.) Link cultivar were grown in a glasshouse under three photoperiod regimes: naturally varying photoperiods (1 2-1 3 h), and fixed photoperiods of 14 and 16 h. Both species behaved as quantitative short-day plants although, in E. utilis, extending the photoperiod from 14 to 16 h delayed head emergence disproportionately. The delay in head emergence did not affect the relative growth rate, and longer photoperiods led to higher shoot weights; main tiller leaf number increased only slightly. The commencement of apical meristem elevation was delayed under longer photoperiods, a feature believed to be beneficial to regrowth. A time-of-sowing study in the field illustrated the effect of this short-day response upon crop development. Sixteen introduced E. utilis cultivars and 10 E. frumentacea cultivars also behaved as quantitative short-day plants. Within each species there were differences between cultivars in sensitivity to photoperiod. Some cultivars of E. utilis were very late and had abnormal head emergence at 16 h. Latitudinal effects on the performance of these cultivars are discussed.