An intensive survey of the freshwater fish and other aquatic fauna of the blackwaters of the North Selangor Peat Swamp Forest and adjacent areas revealed a very high biodiversity, including several new species and new records. 101 species of fish were obtained, about 40% of the known fish fauna of Peninsular Malaysia, of which 47 species were recorded from blackwater sites of the main peat swamp forest and 54 from adjacent areas of the Sungai Bernam and Sungai Tengi. This represents the largest single collection of blackwater species ever known from Peninsular Malaysia. One new genus and four new species of fish (belonging to Lepidocephalichthys, Encheloclarias, Betta and the family Chaudhuriidae), three new records for Peninsular Malaysia (Lepidocephalichthys pristes, Pristolepis grootii, Channa bankanensis) and some 30 new state (Selangor) records are reported. Several fish species, previously regarded as rare, were encountered in large numbers, affirming the value of peat swamps in the conservation of such species. The large number of stenotopic and eurytopic fish species shows that peat swamps are not species-poor as previously assumed. Four amphibians were found, including two rare acid/blackwater species, Pseudobufo subasper and Rana paramacrodon. The presence of decapod crustaceans in highly acidic blackwaters is noteworthy. Four species were obtained. The freshwater crab, Parathelphusa maindroni, previously only known from Sumatra, constitutes a new record for Peninsular Malaysia. Two prawns, one palemonid (a Macrobrachium) and one atyid (a Caridina), are new to science, and are apparently stenotopic acid blackwater species. Nine families and at least 24 species of hemipterans, and 15 genera and five families of coleopterans were collected. Of these insects, about 13 species (eight hemipteran, five coleopteran) are forest species. The gerrid, Crytobates kuiteri, is a new record for Sundaic Southeast Asia. The conservation of this rich aquatic fauna, especially the habitat-specific species should be a major concern and be given utmost priority.