In order to investigate the individual difference in the cocoon quality of female and male silkworm, Bombyx mori, the individual variation of cocoon quality of all the examined population, the variation of cocoon quality within batches. The variation of batches were also examined with 19 batches (843 cocoons of female and 806 of male) of lam 111 X Jam 112, and the daily variation of cocoon quality after harvesting on 300 good cocoons of female and male is as follows：(table omitted) 2. The daily variation of cocoon quality after harvesting. The daily reduction rate of cocoon weight after havesting was higher at the late stage than at the beginning stage of pupal duration and it showed a tendency to be higher in male than in female pupae.