Drosophila obscurin (Unc-89) is a titin-like protein in the M-line of the muscle sarcomere. Obscurin has two kinase domains near the C-terminus, both of which are predicted to be inactive. We have identified proteins binding to the kinase domains. Kin1 bound bällchen (ball, an active kinase), and both Kin1 and Kin2 bound MASK (a 400 kDa protein with ankyrin repeats). Ball was in the Z-disc and M-line of the flight muscle (IFM) and distributed diffusely in the sarcomere. MASK was in both the M-line and the Z-disc. Reducing expression of ball or MASK by siRNA resulted in abnormalities in the IFM, including missing M-lines and multiple Z-discs. Obscurin was still present, suggesting the kinase domains act as a scaffold binding ball and MASK. Unlike obscurin in vertebrate skeletal muscle, Drosophila obscurin is necessary for the correct assembly of the IFM sarcomere. We show that ball and MASK act downstream of obscurin, and both are needed for development of a well defined M-line and Z-disc. The proteins have not previously been identified in Drosophila muscle.