Summry—Immunofluorescence microscopic observations indicated that a monoclonal antibody, Vmp 18, raised against the peptide 199–208 of murine interleukin 1α, cross‐reacted with an antigenic determinant ofDrosophilathorax muscles. Immunoelectron microscopic analysis showed that the gold particles were mainly localized in the Z‐line which is the attachment site of thin filaments from adjacent sarcomeres. On the contrary, the antibody failed to mark the Z‐line in vertebrate skeletal muscle. A Western blot of total protein extract fromDrosophilathorax muscles bound a protein of 43 kDa. Our observations suggest that the Vmp 18 antibody could contribute to clarify the composition of the Z‐line in insect's fl