AT-rich DNA, which constitutes a distinct fraction of the cellular DNA of the archaebacterium Methanococcus voltae, was shown to consist of non-repetitive sequences dispersed on the chromosome and to lack continuous open reading frames in five out of six randomly analyzed cases. Upon subsequent analysis of intergenic regions, AT-rich sequences were again detected. Transcription start points were mapped in front of three open reading frames. The 5' ends of the transcripts were found in variable positions relative to the AT-rich sequences in the different cases. Shine-Dalgarno type sequences complementary to the 3' end of 16S rRNA were discovered at suitable distances from the 5' ends of the genes.