The annual partitioning dynamics of assimilated photosynthates in a low yielding ponkan (Citrus reticulata Blanco cv. Yoshida) and a high yielding cultivar, satsuma mandarin (Citrus unshiu Marc.) were compared. Starch, soluble sugars and hemicellulose in all above ground tissues were monitored at monthly intervals over a period of one year. Ponkan showed a continuous decrease of sugars and starch in all tissues except for mature wood from anthesis (May) until the end of fruit set period (July). Towards fruit maturity and approach of winter, there was a high accumulation of non-structural carbohydrates in the tissues. Maximum carbohydrate content in most tissues was reached in mid-winter (December to Januuary). A close relationship was found between non-structural carbohydrate accumulation patterns in the new leaves and shoots and fruit growth rate indicating that low sink strength of the developing ponkan fruit and inefficient mobilization of non-structural carbohydrates from the current season' s source leaves and mature wood caused low fruit set and yield in ponkan, in contrast to the satsuma mandarin fruit.