The arrangement of repetitive and nonrepetitive DNA sequences in the soybean genome was ascertained by a comparison of the reassociation kinetics of short (250 nucleotides) and long (2700 nucleotides) DNA fragments, the size distribution of S-1 nuclease resistant repetitive duplexes, and a direct assay of the spectrum of DNA sequences present on long DNA fragments enriched in repetitive DNA. These measurements reveal the following: (1) The 1N genome size of the soybean plant is 1.97 pg. (2) Approximately 40% of the soybean genome consists of nonrepetitive or single-copy DNA sequences, while 60% is repetitive DNA. (3) The repetitive DNA is partitioned into three discrete classes termed "very fast," "fast," and "slow," containing DNA sequences repeated an average of 290,000, 2800, and 19 times each. (4) Approximately 35--50% of the soybean genome is arranged in a short-period interspersion pattern of 250 nucleotide slow sequences and single-copy DNA averaging up to 2700 nucleotides in length. (5) From 30% to 45% of the soybean genome is organized into long stretches of repetitive DNA at least 1500 nucleotides in length. (6) Minimal interspersion of repetitive sequence classes occurs in soybean DNA.