Abstract The small crucifer Arabidopsis thaliana has many useful features as an experimental organism for the study of plant molecular biology. It has a four-week life-cycle, only five chromosomes and a genome size less than half that of Drosophila. To characterize the DNA sequence organization of this plant, we have randomly selected 50 recombinant lambda clones containing inserts with an average length of 12,800 base-pairs and analyzed their content of repetitive and unique DNA by various genome blot, restriction digestion and RNA blot procedures. The following conclusions can be drawn. 1. (1) The DNA represented in this random sample is composed predominantly of single-copy sequences. This presumably reflects the organization of the Arabidopsis genome as a whole and supports prior conclusions reached on the basis of kinetics of DNA reassociation. 2. (2) The DNA that encodes the ribosomal RNAs constitutes the only major class of cloned nuclear repetitive DNA. It consists of approximately 570 tandem copies of a heterogeneous 9900-base-pair repeat unit. 3. (3) There is an average of approximately 660 copies of the chloroplast genome per cell. Therefore, the chloroplast genome constitutes the major component of the repetitive sequences found in A. thaliana DNA made from whole plants. 4. (4) The inner cytosine residue in the sequence C-C-G-G is methylated more often than the outer in the tandem ribosomal DNA units, whereas very few differences in the methylation state of these two cytosine residues are detected in unique sequences.