The effect of using the 13C isotope on the ESR signal in a-Si1−x Cx:H is investigated. The ESR linewidth increases with increasing the C content for a-Si1−x Cx.:H with 12C, while it is almost unchanged for a-Si1−x Cx:H with 12C. However, a doublet line which is expected for a C dangling bond having the hyperfine interaction with the 13C nucleus was not detected. In the range of a small x, it is possible that Si dangling bonds with 13C at the back bonds are the origin of the wide ESR signal. In the range of a large x, 13C dangling bonds should exist, but they are in a clustered form, being observed as an exchange narrowed single line instead of the hyperfine split line.