Studies presented here on the fidelity of DNA synthesis in vitro support the hypothesis that a classical base-substitution intermediate (i.e., a misincorporated nucleotide) can yield a frameshift mutation. By using a fidelity assay specifically designed to detect minus-one-base errors, nucleotide substrate pool imbalances that have previously been shown to increase the rate of misincorporation are now shown to also increase minus-one-base frameshift error rates. Examination of the specificity of the errors produced in reactions with various dNTP pool imbalances and various DNA templates revealed that template nucleotides were preferentially lost when they had as a 5' neighbor a nucleotide complementary to the dNTP provided in excess. This suggests that when a misincorporated nucleotide is complementary to the next nucleotide in the template, a misaligned intermediate containing a correct terminal base pair can form and be extended by a DNA polymerase, leading to a frameshift mutation. We present evidence that the proposed mechanism may operate in vivo and discuss the implications of this model for frameshift mutations induced by DNA damage.