Although actin is one of the most abundant proteins found in nature, little detailed information about its molecular structure is available beyond the amino acid sequence. Electron microscopy of negatively stained filaments combined with three-dimensional image reconstruction techniques have revealed the overall size and shape of the actin monomer at 25 A resolution. Higher resolution structural data can be expected from electron microscopy of two-dimensional crystalline arrays and X-ray diffraction analysis of three-dimensional crystals, but only very preliminary results have been reported so far. The original finding by Dos Remedios and Dickens was that skeletal muscle actin forms microcrystals and tubes in the presence of the trivalent lanthanide gadolinium (Gd3+). We have modified and refined their conditions to obtain large crystalline sheets of Acanthamoeba actin and present here a model of the actin monomer in projection to 15 A resolution. We have found that, depending on the ionic strength used, these sheets occur in three different forms: 'cylinders', 'square type' sheets and 'rectangular type' sheets. These different polymorphic forms are built from the same fundamental two-dimensional crystalline actin lattice, which we call the 'basic sheet'. The present concerns the structural analysis of these basic sheets; the crystal polymorphism will be discussed in detail elsewhere (U.A. et al., in preparation). Furthermore, in addition to demonstrating that actin is an elongated globular molecule with a pronounced asymmetric shape in and perpendicular to the plane of the sheet, our results indicate that these crystalline actin sheets might be suitable for three-dimensional structure determination by low-dose electron microscopy of unstained specimens to at least 10 A resolution.