Sucrose density gradient analysis of Neurospora cell free extract showed at least three distinct peaks of enzyme activity; of these, a high molecular weight enzyme was identified as DNA polymerase alpha because of its sensitivity to aphidicolin and to NEM. DNA polymerase mutants of Neurospora crassa were isolated by their resistance to aphidicolin, a specific inhibitor of the eukaryotic DNA polymerase alpha. Some mutants showed an increase in the specific activity of the enzyme. One mutant (E-2-4-1) characterized in detail showed the presence of DNA polymerase which was resistant to inhibitory action of aphidicolin in an in vitro assay. Another mutant (C-3) showed changes in the pH optimum of the enzyme activity. Genetic characterization of the mutants provided evidence for the dominance of the aphr allele controlling aphidicolin resistance and its Mendelian segregation. Some of the aphidicolin resistant mutants were found to be UV-sensitive. Neurospora wild-type and mutant genomic DNA digest was found to hybridize with a cloned yeast DNA polymerase gene. The nick translated yeast DNA polymerase gene was used to screen a genomic cosmid library of Neurospora. A putative clone containing Neurospora DNA polymerase gene has been identified. Further molecular characterization of the Neurospora DNA polymerase gene and enzyme is in progress.