The family Lemuridae consists of two main subgroupings that include the best known and most widespread of the lemurs. The genera Lemur, Eulemur, and Varecia all are considered “true lemurs” and have diets that are largely made up of fruits and leaves. The bamboo lemurs, which include the genera Hapalemur and Prolemur, specialize in eating bamboo although they take other food items as well. Members of the Lemuridae range in weight from 700 g to 4.5 kg. With forelimbs slightly shorter than hindlimbs, they move largely quadrupedally along branches and will leap across gaps in the forest. Typically they live in groups and are active during the day, but all except the ruffed lemurs (Varecia) and the ring-tailed lemurs (Lemur) are also active a certain amount of time during the night, exhibiting an activity pattern referred to as cathemeral (Tattersall, 1987). The extent of this nocturnal activity remains poorly documented in many species, and represents a growing area of interest among primatologists (Curtis and Rasmussen, 2006; Donati et al., 2007; Kappeler and Erkert, 2003). Until the late 1980s, the genus Lemur was considered to include six species and to be closely related to the genus Varecia. However, both Simons and Rumpler (1988) and Groves and Eaglen (1988) suggested that the ring-tailed lemur, Lemur catta, had closer affinities to the bamboo lemurs, Hapalemur, and should therefore be distinguished from other members of the family. As a result, the other species were placed in the genus Eulemur, with only L. catta remaining in the genus Lemur. More recently, Groves (2001) placed the former Hapalemur simus in its own genus, Prolemur, based on a suite of distinctive dental, chromosomal, and behavioral characteristics, and also because its retention in Hapalemur would have made the latter genus paraphyletic (since the sister genus to the Hapalemur griseus group is Lemur, not “H.” simus). We continue to follow this recommendation here, and recognize Hapalemur and Prolemur as distinct genera (Vuillaume-Randriamanantena et al., 1985; Macedonia and Stanger, 1994; Stanger-Hall, 1997). Hapalemur I. Geoffroy, 1851 Bamboo or Gentle Lemurs Members of the genus Hapalemur are best known for a diet dominated by bamboo (Rand, 1935); an unusual ecological specialization among primates. All authorities agree in placing the bamboo lemurs in the family Lemuridae, although considerable uncertainty remains regarding their relationships to other genera in the family. The genus Hapalemur can be readily distinguished from Lemur, Eulemur, and Varecia by features of the head, including its round shape, large face, short muzzle, and ears that are largely hidden by fur. All Hapalemur are medium to small, grayish animals, have moderately long hindlimbs, prefer vertical resting postures, and leap readily between closely-spaced vertical supports. They tend to be crepuscular in their habits. As in Lemur, brachial and antebrachial glands are present (Macedonia and Stanger, 1994). Hapalemur taxonomy, including phylogenetic relationships among members of this genus, is a source of continuing debate (Fausser et al., 2002a, 2002b; Pastorini et al., 2002a, 2002b; Rumpler et al., 2002; Rabarivola et al., 2007). Most authors over the past two decades recognized three species of bamboo lemur: Hapalemur griseus (with at least three subspecies), H. aureus, and H. simus (see Mittermeier et al., 1994). Groves (2001) recently elevated three Hapalemur griseus subspecies to full species status as H. griseus, H. alaotrensis and H. occidentalis, and placed Hapalemur simus in its own genus, Prolemur. He did not, however, recognize the southern bamboo lemur, Hapalemur (griseus) meridionalis Warter et al., 1987, which Fausser et al. (2002a, 2002b) believe is a full species. In this guide, we follow Groves (2001) in moving H. simus to the genus Prolemur and in elevating H. g. griseus, H. g. alaotrensis and H. g. occidentalis to full species, but also follow Fausser et al. (2002) in recognizing H. meridionalis as a full species. In 2007, Rabarivola et al. described two new subspecies of H. griseus (H. g. ranomafanensis and H. g. gilberti). They were described as subspecies rather than as distinct species because available molecular data do not distinguish them. In total, therefore, we currently recognize five species and seven taxa in this genus: H. griseus (with three subspecies), H. occidentalis, H. alaotrensis, H. meridionalis, and H. aureus. However, it is clear that additional research is needed to clarify evolutionary, distributional, and taxonomic relationships in this genus, and also that there may still be new taxa to be discovered.