The Z-band in vertebrate striated muscle links actin filaments of opposite polarity in adjacent sarcomeres to form a regular structure based on a tetragonal lattice. In transverse sections there are two commonly observed appearances of the Z-band seen in different muscles, namely, the small-square lattice and the basketweave forms. A clear example of the latter occurs in the fin muscle of the flatfish plaice and its symmetry is described here. Improved methods over previous work include fast freezing/freeze-substitution and lattice straightening of the scanned images. It is demonstrated here that when a longitudinal section is tilted in the electron microscope about the myofibril axis, the 10 and 01 projections are mirror images of each other about the centre of the Z-band. By examining the symmetry relationships between these views and a longitudinal 11 projection and a transverse view, it is concluded that the symmetry is best described by the two-sided plane group c12. The twofold axis lies in the central plane of the Z-band along the diagonal of the primitive lattice and runs normal to the actin filaments. In contrast, the symmetry of the simple Z-band in fish myotomal white muscle, which in longitudinal sections has the appearance of a single zigzag structure, is p12(1) (Luther, P. K. (1991), J. Cell Biol. 113, 1043-1055). (C) 1995 Academic Press, Inc.