We have applied computational methods to the available database and identified several families of repetitive DNA elements in the Arabidopsis thaliana genome. While some of the elements have features expected of either miniature inverted-repeat transposable elements (MITEs) or retrotransposons, the most abundant class of repetitive elements, the AthE1 family, is structurally related to neither. The AthE1 family members are defined by conserved 5' and 3' sequences, but these terminal sequences do not represent either inverted or direct repeats. AthE1 family members with greater than 98% identity are easily identified on different Arabidopsis chromosomes. Similar to nonautonomous DNA-based transposon families, the AthE1 family contains members in which the conserved terminal domains flank unrelated sequences. The primary utility of characterizing repetitive sequences is in defining, at least in part, the evolutionary architecture of specific Arabidopsis loci. The repetitive elements described here make up approximately 1% of the available Arabidopsis thaliana genomic sequence.