To document the frequency and distribution of repetitive elements in Peromyscus leucopus, the white-footed mouse, a cosmid genomic library was examined. Two thousand thirteen randomly chosen recombinants, with an average insert size of 35 kb and representing 2.35% of the haploid genome ofP. leucopus, were screened with probes representing microsatellites, tandem repeats, and transposable elements. Of the four dinucleotides, (GT)n was present in 87% of the clones, (CT), was present in 59% of the clones, and (AT) n and (GC) n each was represented in our sample by a single clone (0.05%). (TCC)n was present in 8% of the clones. Of the tandem repeats, the 28S ribosomal probe and the (TTAGGG) n telomere probe were not represented in the library, whereas a heterochromatic fragment was present in 9% of the clones. A transposable element, rays, was estimated to occur in 4700 copies, whereas a long interspersed element (LINE) was estimated to occur in about 41,000 copies per haploid genome. LINE and rays occurred together in the same clones more frequently than expected on the basis of chance. Hybridizing the library to genomic DNA from P. leucopus, Reithrodontomys fulvescens, Mus musculus, and human produced general agreement between phylogenetic relatedness and intensity of hybridization. However, dinucleotide repeats appeared to account for a disproportionately high number of positive clones in the more distantly related taxa.