Macronuclear DNA was isolated from Paramecium primaurelia, stock 168. Although the macronucleus is polyploid to the extent of 840C, in other respect the DNA appears to be simply organized, having neither satellite sequences nor substantial amounts of intermediately repetitive sequence. The sequence complexity of macronuclear DNA is quite low for a eukaryote cell, being approximately 19 times more complex than the genome of Escherichia coli. In addition, the GC content is low (25%) and the isolated DNA molecules have lengths mostly in the range 0.2-5 micrometer. In these various respects, the macronuclear DNA of Paramecium is similar to that of other ciliates. A clone of Paramecium cultured under controlled conditions contains polyadenylated RNA sequences which are homologous to 5-8% of the macronuclear DNA. Sequence complexity analysis indicates that the polyadenylated RNA contains two abundance classes of molecules, one present at low frequency and transcribed from approximately 10(4) genes, the other at 100 times greater concentration and transcribed from about 100 genes. The relevance of these results to the control of gene expression in Paramecium is discussed.