Abstract: Sugar contents and activities of sugar-metabolizing enzymes in the peel and juice sacs were analy during the fruit development of Newhall orange(Citrus sinensis Osb．)and Eureka lemon (C．1imon Bunn．f．)．The results showed that，1)changes of the sugar contents and activities of sugar-metabolizing engynlL were diferent in citrus cuhivars and in diferent fruit parts；2) the reduction sugar content was higher than the sucrof content in both peel and juice sacs of om ge or lemon，as was diferent from other’s study；3)the continuous decrease of the soluble acid invertase(SAI)activity during sugar accumulation of orange juice sacs，close to zero at the fruit—harvesting time， implicated that the decline of SAI activity was likely related to sugar accumulation；and 4)the phenomena that the activity of the alk~ine inverse(NI)in lemon juice sacs decreased continuous with fruit ripeness and kept the lowest level at the fruit—harvesting time while as the activity of SM reached a higher level，up to 6 times of that of the other enzymes，indicated that the consumption of sugars in lemon juice sacs was related to NI activities during the fruit developmental stages and to SM at fruit maturing period．Sucrose synthase (SS)and invertase might play a role in sucrose metabolism in citrus fruit juice sacs considering the significant diference of sugar contents between in orange and lemon juice sacs as well as the changes of their related crose—
metabolizing enzymes．

Key words: Citrus, Fruit, Sugar, Sucrose—metabolizing enzyme