The frequency of movement of Tc1, a 1.6-kilobase transposable element in the nematode Caenorhabditis elegans, is under genetic control, and Tc1 insertion sites are widely but nonrandomly distributed. The usually high frequency of insertions at multiple sites in the gene unc-22 suggested that this gene might be particularly rich in preferred target sites. To discover the features of Tc1 target sites, we have sequenced the sites of seven independent Tc1 transpositions into unc-22 and three other sites. Our comparison of these and two other sites from the literature indicates that in all cases Tc1 integrates at the dinucleotide T-A when it is flanked both 5' and 3' by particular preferred nucleotides. Our analysis revealed the following consensus target for Tc1 integration: G-A-K-A-T-A-T-G-T, in which K = G or T. This target site sequence specificity has implications both for the mechanism of Tc1 transposition and the use of Tc1 in cloning genes by transposon-tagging.