SUMMARY This paper reports the results of experiments carried out in Surrey and Cardiganshire (lat. 51o 19′ N. and 52o 25′ N.) and a t Werribee in Victoria (lat. 37o 54′ S.) designed to test some Australian and British wheats grown under various periods of illumination. Light of weak intensity (4–5 c.P.) was employed to prolong the daily period.2. Generally the Australian varieties were “earlier” at all centers under all periods of light than the British. A sharp distinction between the reaction of spring and winter varieties was observed under short periods of light. Retardation or omission of floral development was always accompanied by prolonged vegetative vigour. The influence of such treatments upon the longevity of the plants was observed. Observations are made upon the relationship of the photoperiodic response to the geographic source of varieties. The results are very briefly considered from the standpoint of the C/N hypothesis and the utilisation of the products of photosynthesis. Certain economic aspects of the work are indicated.