Fiber systems of fertilized eggs of the sea urchin Strongylocentrotus purpuratus become aggregated and thus visible in phase-contrast light microscopy, when cells are fixed in 2% glutaraldehyde in 0.45 M Na-acetate buffer at pH 6.0 and embedded in epoxy. Studies of whole mounts and of 1-μm stained sections of the first-division cycle revealed a spiral array of subcortical fibers that apparently grow inward from the cell surface shortly after sperm entry and disappear prior to streak stage. They are independent of the microtubule system associated with the sperm aster, amphiaster, and mitotic apparatus. Their chemical identity is not known, but they may very likely be actin.