The occurrence of watercore in 'Orin' apples (Malus pumila Mill. var. domestica Schneid.) was prevalent from late August to late September ; there was a significant correlation between watercore rating and maximum fruit temperature during late August. The watercore occurred in immature apples long before the increase of ethylene and 1-aminocyclopropane-1-carboxylic acid (ACC), indicating that the watercore observed in this study was early watercore. Membrane permeability increased from mid-August to mid-September and showed a significant correlation with watercore. The degree of watercore during late August was significantly correlated with flesh firmness, soluble solids content (SSC), total sugars, fructose, glucose in the inner flesh, and sorbitol but not with titratable acidity, starch, and sucrose. The seasonal changes in sorbitol content paralleled the incidence of watercore ; the coefficients of correlation between them were 0.881 and 0.827 for the inner and outer flesh tissue, respectively. These results indicate that the changes in physiological factors such as membrane permeability and sorbitol content in the fruit during summer play an important role in inducing early watercore in apples. We propose that the physiological mechanism of early watercore development in immature apples is similar to that of high temperature-induced watercore during maturation but differs from that of low temperature-promoted late watercore.