The adsorption of lambda-phage DNA onto mica was investigated with atomic force microscopy. We found that the morphologies depended on the solvent conditions in the sample preparation procedure. Flat-lying networks of hybridized single-stranded DNA were obtained if ultrapure water was used. If buffered conditions are maintained during the whole of the preparation procedure, single double-stranded DNA molecules are adsorbed. The adsorbed double-stranded DNA molecules subsequently can be condensed in situ on the surface by a brief rinse with anhydrous ethanol in the presence of divalent magnesium cations. The majority of these surface-directed and ethanol-induced condensed structures are toroids, but a small fraction of rods also has been observed. Analysis of the height and lateral dimensions shows that the toroids are single-molecular and disk-like with a height of one to two DNA diameters. The thin toroid morphology appears to be a general phenomenon of surface-directed condensation, irrespective of the nature of the condensing ligands and the specific surface interaction.