Giant smooth muscle fibers have been found in the marine planctonic invertebrate Beroe ovata and studied by electron microscopy. These cylindric multinucleated fibers can reach 40 μm in diameter and several centimeters in length. Myofilaments run parallel to the main axis of the cell and make up a sheath around the axially located cell organelles. Two types of myofilaments are consistently observed; thin filaments (actin like), 6.2 ± 0.04 nm in diameter, rosettes of 8 to 12 filaments around thick filaments (myosin like), 16.4 ± 0.08 nm in diameter. Dense bodies and attachment plates are totally lacking; thin filaments are supposed to attach directly upon the sarcolemma. The sarcoplasmic reticulum forms a longitudinally oriented network of branched tubules among the myofilaments; its relative volume has been estimated from cross sections to be 1% of the myofilament volume. Couplings of the sarcoplasmic reticulum with the sarcolemma are seldom observed, except at intermuscular junctions where they are always present. Tubular and sac-shaped invaginations of the sarcolemma have been observed and are thought to be analogous to the caveolae of invertebrate and vertebrate smooth muscles. Mitochondria are arranged in large clumps. They often bear double-walled, cristae-free tubular appendages, which have been called mitochondrial tubules. Adjacent muscle fibers are sometimes linked by close junctions. Neuromuscular junctions occur frequently and have been described in detail elsewhere.