DNA synthesis has been investigated in Amoeba proteus by pulse-labelling cells of known age with [H]thymidine. Ninety per cent of the DNA so labelled was synthesized during the first quarter of the cell cycle. Synthesis was in two peaks: the first occurring between 0-5 and 4 h after division and the second at about 10 h. All cells labelled at both peaks. The authenticity of the second peak was proved statistically. Considerable variation was observed among amoebae of similar age. In experiments in which daughter amoebae underwent different treatments, differences in the rates of incorporation of [H]thymidine due to differences in the nutrition of the cells were found to be a predominant cause of variation. Heavy feeding reduced labelling; starvation increased labelling while at the same time reducing variability.