Linear dichroism (LD) gives useful information on the interaction between DNA and the directly acting carcinogen N-acetoxy-N-2acetylaminofluorene (AAAF). In 50% methanol solvent with low ionic strength only a weak complex (van der Waals) appears. However, above 40 degrees C strand separation takes place and a covalent aminofluorene complex forms. After renaturation a characteristic positive LD band is observed at 306 nm. The average angular orientation of the long-axis of the fluorene moiety (47 degrees to the local helix axis) is inconsistent with intercalation. It can be explained for instance by a free rotation around a C(DNA)-N(aminofluorene) bond or by a major groove site. The occupation density was 1--2 aminofluorene residues per 100 bases. With native DNA, AAAF slowly forms a covalent complex which has a negative LD at 307 nm. The orientation (70--90 degrees) is consistent with steric direction by the strand.