Past field studies on nocturnal primates in Africa have focused on species occupying woodland in southern Africa and primary rain forest in West and Central Africa. Relatively little is known about the galagos (Primates, Galagidae) inhabiting the fragmented, evergreen forests of East Africa. The development of new field techniques, coupled with technological advances has enabled more accurate identification of species based on species-specific calls. Nocturnal surveys during five periods of field work (July-August, 1990; August-November, 1992; February-May, 1993; July-November, 1993; July-November, 1994) in isolated coastal and Eastern Arc Mountain forests in Tanzania indicate that considerable speciation has taken place among galagos in these forest 'islands'. The loud calls of galagos, recorded during surveys and computer analysed, provide a vocal profile that is consistent within species and which can be used in a taxonomic context to identify species. This approach, founded on an examination of a major element of the Specific-Mate Recognition System (Recognition Concept of Species: Paterson, 1985), has resulted in the identification and naming of 4 new species of galago in Tanzania (Galagoides granti, G. udzungwensis, G. orinus, G. rondoensis). The identification of these species is supported by evidence of differences in other aspects of vocalisations as well as in reproductive and hair morphology and lifestyles. These discoveries suggest the true number of species of both galagos and other secretive, nocturnal, forest-dwelling mammals may have been seriously underestimated. At the start of this study 11 galago species were recognised (Nash et a1., 1989), by the end this has increased to at least 17. These species are placed in a new classification of the family Galagidae, based on vocal profile differences. An assessment of the conservation threat status of Tanzanian galago indicates that the 4 new species are all of at least IUCN Red List Vulnerable status. This is primarily due to restricted distribution in small, fragmented and isolated habitats which themselves are under considerable threat. Recommendations are made concerning the conservation of galagos and their habitats. Increasing knowledge of the high levels of endemism in the evergreen forests of Tanzania further highlights the biological value of these areas in a region where considerable conservation effort is often restricted to savannah habitats and their large mammal occupants.