The fruits of two cultivars, 'Ohmishima Navel' (Citrus sinensis Osbec.) and 'Hassaku' (C. hassaku ex Tanaka), were analyzed to obtain the basic data for elucidating the physiological mechanisms of fruit maturation in medium to late ripening citrus. The changes occurring in sugars and acids in the fruits of the two cultivars during growth and ripening were demon-strated in the pulp and peel separated into albedo and flabedo. 1. Most of acid was free in the pulp, while in the peel most of it was combined. The inflection points of the peel extracts ranging from pH 6.2 to 6.4 were lower than those of the pulp extracts. This indicates the presence of free oxalic acid in the peel. 2. In the peel of the two cultivars, malic, oxalic and acetic acids were the predominant ones, but citric acid being the predominant one in the pulp was low in concentration. In the albedo of 'Hassaku’, however, the amount of citric acid was higher than those of the three acids. 3. Comparing the present results with those of 'Miyagawa Wase' satsuma, the difference in the metabolism of fruits between early ripening and rate ripening citrus was brought to light; i. e., early occurence of the period of metabolic conversion and the rapid changes of chemical constituents in the growing fruits were considered to result in early maturity 4. In the pulp of the two cultivars, contents of reducing sugars were higher than those of non-reducing sugars and the reverse was not detected up to early January. This indicates that the pulp has not completely ripened and that postharvest ripening of the fruits is required.