Bacteriophage PX174 viral strand DNA molecules shorter than genome length found late in the infectious cycle in Escherichia coli were 5' end labeled with 32p. Hybridization of the 32P-labeled molecules to restriction enzyme fragments of 4X replicative form DNA revealed an excess of XX molecules whose 5' ends mapped in HaeIII fragments Z3 and Z4 in comparison with fragments Zl and Z2. This suggests that initiation of fX174 viral strand DNA synthesis may occur at internal sites on the complementary strand. There are several appropriately located sequences that might serve as n' (factor Y) recognition sequences and thereby facilitate discontinuous synthesis of the viral strand.