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1. The G + C content of the DNAs of Amoeba proteus and Amoeba indica was determined by buoyant density gradient centrifugation. 2. Generally, different clones of the same strains did not differ in the G + C content of their main DNA or their satellite DNA. 3. The G + C content of the main DNA of the TD and XD strains of A. proteus were the same (25.3 and 29.3% respectively) as were the G + C contents of the satellite DNA (54.7 and 61.8% respectively). 4. The main and satellite DNAs are reversed for the F strain of A. proteus and the I strain of A. indica; the G + C contents of the main DNA being 57.4 and 59.8% respectively and those of the satellite DNA being 23.5 and 28.4% respectively. 5. The heterologous hybrids produced by nuclear transplantation between the A strain of A. proteus and the I strain of A. indica have the same G + C content in their main and satellite DNAs as do their cytoplasmic parents. 6. The data suggest that the occurrence of phenotypic variations does not necessarily mean an alteration in the G + C content of either main or satellite DNA. 7. There is also evidence that the G + C content of these DNAs in under cytoplasmic control.