Description: Samples with vein structures were taken from Sites 787 and 793 in the forearc basin of the Izu-Bonin island arc off Aoga Shima and Sumisu Jima, respectively, between the present volcanic front and the outer arc high. The samples were studied by thin section, X-ray radiograph, and magnetometer; they are discussed with respect to the tectonic implication of the vein structures to the island-arc development. Vein structures are developed in finer, more clayey, preferentially radiolarian-bearing mudstone, subvertical to the bedding plane, which is mostly horizontal. The veins are restricted to certain horizons: in the upper Oligocene at Site 787 and in the lower Miocene at Site 793. The veins are filled with a dominant clay mineral (montmorillonite), which flowed into the vein when the fracture and concomitant stress drop occurred. Some clay mineral was deposited from the fluid that invaded the vein. Some veins might have occurred as hydraulic fractures. The shape, mode of occurrence, and other structural features indicate that the veins originated either as extension fractures or shear cleavages, and then were rotated by the following shearing parallel to the bedding. Sometimes the bedding-parallel slip planes are dislocated by the veins, and sometimes vice versa. This suggests that the vein formation and bedding parallel slip alternately occurred within the same stress environment. Vein attitude was measured by a magnetometer, after alternating field demagnetization; we interpret that they originally formed as subvertical planes, the trends of which average to N45W. The quantity of samples studied was small, but the trends suggest that the stress field for veining might have had a relative extensional stress axis that lay subhorizontally and trended generally northeast. This stress orientation might be attributed to either bending or normal faulting in the forearc basin, at a time when the arc trended northwest.