Zasp family proteins Zasp52 and Zasp66 directly interact with alpha-actinin (Actn), colocalize at the Z-disc, and play a crucial role in myofibril assembly, the last step of muscle development. In live imaging, GFP-Zasp52 forms steadily growing Z-bodies initially evenly dispersed in myotubes, which sort out to future Z-disc locations and eventually fuse to form a contiguous Z-disc. Here we show the molecular mechanism of Zasp function. Genetic experiments demonstrate that Zasp52 and Actn are both haploinsufficient genes whose gene products are required at similar stoichiometry at the Z-disc. Next, pull-down and chemical crosslinking assays show that Zasp52 dimerizes and heterodimerizes with Zasp66, likely explaining the cooperation of Zasp52 and Zasp66 and the steady growth of Z-bodies during myofibril assembly. Finally, we demonstrate through pull-down and cosedimentation assays as well as direct visualization that Zasp52 binds and bundles F-actin. Genetic interaction data indicate that binding of Zasp52 to actin thin filaments is crucial for indirect flight muscle development.