In the light of the fact that Cryptorrhynchus lapathy Linne (Coleoptera: Cur-culionidae) is an important past of poplar in korea the Bionomics of the insect were studied to get some basic informations for controlling the insect. The results obtaind are as follows; 1. The poplar and willow borer has one generation in a year overwintering with egg. 2. The pupation stage was from the middle Julne to the middle July. 3. The adult appears from the early July to the middle August. 4. The average length of body of adult was about 8.5㎝ with female, 7.9㎝ with male and the length of pupa was about 11.3㎝. 5. The average longevities of adults were about 30 days with mate and 40 days with female. 6. The length of damage hole were about 10.3㎝. 7. The egg-laying period was from July to August.