The fruits of two cultivars, 'Miyagawa Wase' (Citrus unshiu Marc.) and 'Miyauchi Iyokan' (C. iyo hort. ex Tanaka), were analyzed to obtain the basic data for elucidating the physiological mechanism of fruit maturation in Citrus. The changes occurring in sugars and organic acids in the fruits of the two cultivars during growth and ripening were demonstrated in the pulp and peel. 1. The highest value of acid content in the pulp per fruit was detected at late August in 'Miyagawa Wase' and at late December in 'Miyauchi Iyokan'. In 'Miyagawa Wase', decrease of total acid concentration in the pulp was dependent on decrease in absolute amount of total acids and dilution effect due to fruit growth, whereas in 'Miyauchi Iyokan' it was dependent on the fact that absolute amount of total acids increasing was less than that of total acids decreasing on account of dilution effect due to fruit growth 2. Most of acid was free in the pulp, while in the peel most of it was combined. The inflection points of the peel extracts ranged from pH 6.2 to 6.5 and were lower than those of the pulp extracts. This indicates the presence of free oxalic acid in the peel. 3. In the peel of the two cultivars, malic, oxalic and acetic acids were the predominant one in the pulp was low in concentration 4. In the pulp of the two cultivars, concentration of reducing sugars rapidly increased from October and was, subsequently, very higher than that of non-reducing sugars. In a late stage of fruit growth, concentration of non-reducing sugars decreased in the peel of 'Miyauchi Iyokan', but did not decrease in the peel of 'Miyagawa Wase'.