Sound signals of 9 species of rainforest tree squirrels were studied in the field and in captivity at Makokou, Gaboon. The signals can be classified into 7 behavioral categories: nestling ticking and isolation, distress, contact-seeking, defensive, low intensity alarm, and high intensity alarm. On the basis of physical structure, sounds are further classified into 12 basic types and their variants. A given species has a repertoire of about 7 of these sound types. Review of the literature on sciurid calls suggests that most species emit sounds under the same set of behavioral contexts described for the African squirrels, and that many of the basic structural sound types are widespread within the same contexts throughout the group. In comparison to temperate squirrels, some calls of rainforest species show a lowering of frequency and increase in length. These features are apparently adaptations to the dense vegetation and high background noise of the rainforest environment.