Flagellar structure in spermatids of several species of cricket was studied with the electron microscope. The flagella of mid-spermatids contain the usual 9 + 2 set of fibers and a set of nine accessory fibers. At first all are hollow, then the lumina become filled with an electron-opaque matrix material in which narrow electron-lucent microcylinders are embedded. The accessory fibers and one central fiber become filled first, then the B subfibers and the other central fiber, and finally the A subfiber. In all but the B subfibers, microcylinders are arranged in a circular or oval group that lies against the wall of the lumen and encloses one or several additional microcylinders. In accessory fibers there are 9–11 microcylinders in the outer group and 4–5 in the inner group. In the central fibers and the A subfibers there are 7–9 microcylinders that enclose one or two more. In the B subfibers there is a crescentic group of 6–7 microcylinders that partially encloses 2–3 more. Microcylinders become packed as though they are independent units; the matrix appears to be an amorphous substance that fills the spaces around the microcylinders. The mean diameter of the microcylinders is 36 A, and they have a center-to-center distance of 56 A. In both respects they resemble wall subunits of flagellar fibers and microtubules and they may be similar structures but with a different localization. The diameter of accessory fibers is about 350 A, which is 25% greater than that of the other flagellar fibers and of cytoplasmic microtubules. Rotation tests suggest that the accessory fibers have 16 wall-subunits.