The Hylobatidae (gibbons) are among the most endangered primates and their evolutionary history and systematics remain largely unresolved. We have investigated the species-level phylogenetic relationships among hylobatids using 1257 bases representing all species and an expanded data set of up to 2243 bases for select species from the mitochondrial ND3-ND4 region. Sequences were obtained from 34 individuals originating from all 12 recognized extant gibbon species. These data strongly support each of the four previously recognized clades or genera of gibbons, Nomascus, Bunopithecus, Symphalangus, and Hylobates, as monophyletic groups. Among these clades, there is some support for either Bunopithecus or Nomascus as the most basal, while in all analyses Hylobates appears to be the most recently derived. Within Nomascus, Nomascus sp. cf. nasutus is the most basal, followed by N. concolor, and then a clade of N. leucogenys and N. gabriellae. Within Hylobates, H. pileatus is the most basal, while H. moloch and H. klossii clearly, and H. agilis and H. muelleri likely form two more derived monophyletic clades. The segregation of H. klossii from other Hylobates species is not supported by this study. The present data are (1) consistent with the division of Hylobatidae into four distinct clades, (2) provide the first genetic evidence for all the species relationships within Nomascus, and (3) call for a revision of the current relationships among the species within Hylobates. We propose a phylogenetic tree as a working hypothesis against which intergeneric and interspecific relationships can be tested with additional genetic, morphological, and behavioral data.