We have sequenced amplified products of 44 short tandem repeat alleles at the human D11S554 locus. The sequences revealed four distinct, but related, families of consensus repeat patterns. Six alleles deviated from the consensus patterns. Alleles differed in size by as little as one base pair, and precise sizing is required for analysis of alleles at this highly polymorphic locus. The evolution of alleles at the D11S554 locus and related short tandem repeat loci is discussed.