A factor which inhibited DNA polymerase [EC 184.108.40.206] activity was isolated from the cytoplasm of plasmodia of true slime mold, Physarum polycephalum. This factor was purified by DEAE-Sephadex and CM-cellulose column chromatographies, heat treatment and gel filtration. This inhibitor was heat-stable, insensitive to trypsin [EC 220.127.116.11] and was not digested by RNase [EC 18.104.22.168] or DNase [EC 22.214.171.124]. The molecular weight was 16,000 as determined by gel filtration, and the isoelectric point was determined to be pH 10.1. In the presence of the inhibitor, Km for DNA in the DNA polymerizing reaction was markedly increased. The inhibitory effect was eliminated by addition of excess DNA, but the addition of excess enzyme or deoxyribonucleoside triphosphates had no effect on the inhibition.