In order to delay flowering of early clones and advance that of late ones, to permit otherwise unobtainable crosses, various photoperiod treatments were investigated. The most effective treatment to delay flowering was that in which natural induction was prevented by a night light break regime (NLB) for a predetermined duration followed by a daylength extension imitating natural inductive daylengths. Clones responded to a 10 min difference between inductive daylengths. When the NLB was followed by natural days (shorter than normal for induction) all stages of development were hastened and one clone was rendered male sterile. Periods of NLB after floral initiation in general delayed development; applied at the spikelet initiation stage, flowering of clones was delayed by 1–2 weeks and pollen fertility was reduced, but applied at the earlier stage of branch initiation, flowering of some clones was delayed whereas inflorescences of the remainder did not emerge. Flag leaves of the latter did eventually emerge and were significantly longer than those of control plants, indicative of reduced competition with the inflorescence as a result of the developmental delay. Advance of flowering in a late clone was effected by shortening daylengths by 30 min (SD) for various periods prior to and during floral development and, although the greatest advance was afforded by the continuous SD treatment, most of those for lesser periods also reduced in earlier flowering and therefore offer a more economic use of dark-room facilities when advancing late clones. The precise photoperiod control of successive developmental phase is discussed and it is conducted that sugarcane is a quantitative intermediate day plant, and that successive developmental stages have shorter less critical daylength requirements. This hypothesis is discussed in relation to flowering behaviour of sugarcane at different latitudes.