Although some mitochondrial, X chromosome, and autosomal sequence diversity data are available for our closest relatives, Pan troglodytes and Pan paniscus, data from the nonrecombining portion of the Y chromosome (NRY) are more limited. We examined Ϸ3 kb of NRY DNA from 101 chimpanzees, seven bonobos, and 42 humans to investigate: (i) relative levels of intraspecific diversity; (ii) the degree of paternal lineage sorting among species and subspecies of the genus Pan; and (iii) the date of the chimpanzee͞ bonobo divergence. We identified 10 informative sequencetagged sites associated with 23 polymorphisms on the NRY from the genus Pan. Nucleotide diversity was significantly higher on the NRY of chimpanzees and bonobos than on the human NRY. Similar to mtDNA, but unlike X-linked and autosomal loci, lineages defined by mutations on the NRY were not shared among subspecies of P. troglodytes. Comparisons with mtDNA ND2 sequences from some of the same individuals revealed a larger female versus male effective population size for chimpanzees. The NRY-based divergence time between chimpanzees and bonobos was estimated at Ϸ1.8 million years ago. In contrast to human populations who appear to have had a low effective size and a recent origin with subsequent population growth, some taxa within the genus Pan may be characterized by large populations of relatively constant size, more ancient origins, and high levels of subdivision.A great deal has been learned about the evolutionary history of humans and great apes through comparisons of homologous DNA sequences. Humans are thought to be most closely related to chimpanzees (Pan troglodytes) and bonobos (Pan paniscus), with whom we share Ϸ99% of our genomic sequence (1). Much is also being learned about levels of genetic diversity within humans through comparisons of allelic variation at different loci in the human genome (2). Studies of intraspecific diversity in our closest living relatives, however, have lagged far behind those of humans. Comparisons of levels of intraspecific variability have important implications for understanding the processes involved in speciation and clarifying the demographic history of contemporary populations of humans and great apes. Because chimpanzees and bonobos have such a poor fossil record, genetic data can play a major role in elucidating the evolutionary history of the genus Pan.Currently, there are comparative human-chimpanzee sequence data for loci representing three of the four compartments of the genome: the autosomes, the X chromosome, and mtDNA. Comparisons of mitochondrial sequence variation in chimpanzees, bonobos, and humans are consistent in showing 2-to 4-fold higher levels of diversity in both Pan species than in humans (3-6). Studies of autosomal and X-chromosome loci have generally shown higher levels of nucleotide variation in Pan, with some exceptions. In a study of a Ϸ1-kb region of the HOXB6 gene from 105 humans, 45 chimpanzees, and 19 bonobos, Deinard and Kidd (7) found more than three times as much nucleotide diversit...