The synthesis of DNA in two hypotrichous ciliates, Styx sp. and an amicronucleated strain of Oxytricha sp., was studied by high voltage (1000 kV) electron microscopy. High voltage EM permits use of thick sections (0.25-0.40 micron), including serial sections; thick sections produce strong autoradiographic images with relatively short exposure times. The autoradiographs show that DNA synthesis occurs in a narrow part of the rear zone of a replication band in the macronucleus. Macronuclear DNA synthesis occupies a substantial part of the interdivision interval, and micronuclear DNA synthesis in Styx sp. takes place in early prophase at a time when macronuclear DNA synthesis is in its terminal phase.