The ability of tendon to withstand tensile forces is largely attributable to an extracellular matrix containing parallel collagen fibrils organized into fascicles. A major belief is that force is transmitted between collagen fibrils via interactions of molecules at the fibril surface. However, there is existing evidence (reviewed here) for persistent connections between fibrils formed by interfibrillar fusion. Furthermore, in vitro studies have shown the ability of the ends of fibrils to fuse together. In this study, we show using serial section electron microscopy of embryonic mouse-tail tendon further evidence for interfibril fusion in vivo. We showed: (1) fibrils fused via Y-shaped branches without disruption of the 67 nm D-periodicity, (2) the frequency of the branches was approximately 1:20 000 D-periods, and (3) the small angle of the Y ranged from 4 degrees to 10 degrees, indicating a structure-based mechanism of branch formation. The regular occurrence of Y-shaped branches between collagen fibrils suggests direct force transmission between fibrils. Furthermore, the formation of the Y-shaped branches by tip-to-shaft fusion would explain the paucity of fibril tips in vivo.