Fiber cells isolated by mechanical disruption of the tissue in Ca2+-free sea water attach firmly to the substrate by discrete adhesion plaques. They are capable of forming a lamellipodium and long, slender extensions while the cell bodies remain stationary. The extensions are slowly elongated but can suddenly be withdrawn by contraction. Bundles of F-actin are attached with their “plus” ends close to the tips of the extensions. Within the cell body, microfilaments form a loose cortical layer and criss-crossing patches. Microtubules are not present in newly formed extensions but seem to stabilize “older” extensions. In these, they are bundled by crossbridges and oriented lengthwise. In the cell body, 37 nm “macrotubules” are found as well as the prevailing microtubules 24 nm in thickness. They radiate from an organizing center close to the mitochondrial complex, which can, however, also give rise to normal microtubules. Since this organizing center seems to nucleate either macrotubules or microtubules, but never both at the same time, it is speculated that it may exist in two alternative states.