The DNA sequences of 185 independent spontaneous frameshift mutations in the rIIB gene of bacteriophage T4 are described. Approximately half of the frameshifts, including those at hot spot sites, are fully consistent with classical proposals that frameshift mutations are produced by a mechanism involving the misaligned pairing of repeated DNA sequences. However, the remaining frameshifts are inconsistent with this model. Correlations between the positions of two base-pair frameshifts and the bases of DNA hairpins suggest that local DNA topology might influence frameshift mutation. Warm spots for larger deletions share the property of having endpoints adjacent to DNA sequences whose complementarity to sequences a few base-pairs away suggest that non-classical DNA misalignments may participate in deletion mutation. A model for duplication mutation as a consequence of strand displacement synthesis is discussed. In all, 15 frameshifts were complex combinations of frameshifts and base substitutions. Three of these were identical, and have extended homology to a sequence 256 base-pairs away that is likely to participate in the mutational event; the remainder are unique combinations of frameshifts and transversions. The frequency and diversity of complex mutants suggest a challenge to the assumption that the molecular evolution of DNA must depend primarily upon the accumulation of single nucleotide changes.