Observations and histological studies ou the reproductive cyole of sea cucumber(Stichopus japonicus Selenka) were made in Dalian coast, and the results are asfollows: 1. The bighest value of gonad index, value R and biggest primary branch diameterof gonad are found from July to middle of Angust. The spawning indivaduals in thisperiod account for 85--100% of total spawning indiveduals in a year. 2. As to histology, the development of gonad can be divided into 5 stages: rest,Proliferation, growth, maturation and spawhing periods. The period of tho spawningpeak varies with tbe size of the indivdual and locality. The bigger individual inshallow water shows a tendancy to spawn earlier. 3. From the seasonal varfation of the net weight of digostive tract and ite con-tents, which revealod that the period of asetivation of the sea cuoumber in Dalian coastsbegan from August to October. During aestivation period, its intestine atrophiesextremely and it stops feeding. It will begin moving and feeding by the end of October,or the early November