The spinning behavior of the silkworm, Bombyx mori, was recorded on videotapes from two angles and analysed by three dimentional computer graphics using the Japanese (J. 124), Chinese (C.124) and their hybrid (J.124 × C.124) strains. These strains constructed typical peanut-shaped, spherical and ellipsoidal cocoons, respectively. Linear representation of the spinning posture revealed that larvae fixed the posterior half of the larval body (6th to 13th segment) and spun silk moving their anterior half (1st to 5th segment) for the most spinning period in all strains used. Little difference was observed in the average spinning speed among them. The Japanese strain spun primarily in a S-letter posture and changed its direction frequently. The larva of Chinese strain often assumed a C-letter posture and showed directionchanging behavior with comparatively lower frequency. The hybrid larva threw the head back largely in an U-letter shape during most of the spinning period and showed cocoon expansion behaviors most frequently. The cocoon expansion behavior occurred mainly at both ends of the peanut-shaped cocoon (J.124), at the center part of the spherical cocoon (C.124) and at both shoulders in the ellipsoidal cocoon of the hybrid strain. Thus, there exist strain-specific features in the spinning behavior, and it is suggested that the main behavioral factors affecting cocoon shape formation are the spinning posture and the cocoon expansion behavior during spinning.