The sequence organization of nuclear DNA in the single-celled alga Euglena gracilis has been studied by a combination of techniques: (1) the comparison of the reassociation kinetics of DNA fragments 300, 2000 and 8100 nucleotides long; (2) the reassociation of 32P-labeled DNA fragments of various lengths with driver fragments 300 nucleotides long; (3) the hyperchromicity of DNA structures formed by the reassociation of repetitive sequences; (4) and the direct measurement of the size of the duplex regions of reassociated repetitive DNA resistant to S1 nuclease. The single copy DNA sequences are approximately 1500 nucleotide pairs long and are interspersed with repetitive DNA sequences. The repetitive DNA, consisting of both highly repetitive and middle repetitive sequences, consists of one fraction of nucleotide sequences (0.67) with an average size of 4900 nucleotide pairs and a second fraction (0.33) with an average size of 1000 nucleotide pairs, 34% of the DNA consists of foldback sequences which are present on 45% of the DNA 4000 nucleotides long.