A study of the relative utilization of thymine and thymidine as precursors for DNA synthesis during normal growth in Bacillus subtilis showed that thymine serves preferentially as a precursor for ;repair' synthesis, whereas thymidine is used preferentially for ;replicative' synthesis. Further, evidence was obtained which suggests that during normal growth both ;replicative' and ;repair' DNA syntheses occur simultaneously. ;Repair' synthesis is distinguished not only on the basis of its preferential utilization of thymine but also by its selective inhibition by caffeine. ;Replicative' synthesis, however, is selectively inhibited by 6-(p-hydroxyphenylazo)-uracil. ;Repair' synthesis would seem to be a ;pre-fork' phenomenon and its inhibition is highly lethal to the cell.