Chlorella NC64A cells infected with the large double-stranded DNA-containing virus PBCV-1 and uninfected cells were assayed for DNA polymerase activity. Both uninfected and infected cells contained three forms of DNA polymerase activity: (i) an exogenous DNA-dependent 10,000 g soluble fraction, (ii) an exogenous DNA-dependent 10,000 g particulate fraction, and (iii) a DNA-independent 10,000 g particulate fraction. The three DNA polymerase activities in the infected and uninfected cells were distinguished from one another by the conditions required for optimum activity and by their sensitivity to inhibitors.