This study examined two aspects of cyanogenesis in Brombya platynema F. Muell. (Rutaceae), a subcanopy tree endemic to tropical rainforest in far north Queensland, Australia. First, cyanogenic glycosides in foliage were fractionated and identified. The rare meta-hydroxylated cyanogenic glycoside, holocalin, was identified as the principal cyanogen, and traces of prunasin and amygdalin were detected. This is the first characterisation of cyanogenic constituents within the genus, and to the authors’ knowledge, only the third within the Rutaceae, and the order Rutales. Second, variation in cyanogenic glycoside content within a population of B. platynema in lowland tropical rainforest was quantified. Both qualitative and quantitative polymorphism for cyanogenesis was identified. Interestingly, ~57% of individuals were considered acyanogenic, with concentrations of cyanogenic glycosides less than 8 μg CN g^–1 DW. Among cyanogenic individuals there was substantial quantitative variation in cyanogenic glycoside concentration, which varied from 10.5 to 1285.9 μg CN g^–1 DW. This high frequency of acyanogenic individuals is contrasted with the apparent absence of the acyanogenesis among populations of other tropical rainforest tree species. In the high herbivory environment of the tropical rainforest, this frequency of acyanogenesis among cyanogenic tropical tree taxa is unique.