Transverse ultrathin sections and the freeze-fracture deep-etch method were used to study the structure of vertebrate thick filaments. We examined intact frog semitendinosus and glycerinated rabbit psoas muscle fibers. In transverse thin sections, using standard staining methods, cores were moderately well revealed. The cores were particularly distinct when sections were stained with potassium permanganate followed by differentiation with bleach. For freeze-etching, the specimens were generally fractured transversely, and occasionally obliquely, to filament axis, and were then deep-etched and rotary-shadowed. In both transverse and oblique images, thick filaments were made up of a central core, surrounded by an annular region. Since cores are characteristic of invertebrate thick filaments, it is tempting to suggest a similarity. On the other hand, the cores observed here seem simplest to interpret as being hollow, as in microtubules.