The biochemical and genetical relationship between aliphatic glucosinolates which have methylthioalkyl, methylsulphinylalkyl and alkenyl side chains has not been resolved by biochemical studies. In this study, two hypothetical models are tested by the genetic analysis of a backcross population between Brassica drepanensis and B. atlantica. The results support one of the models in which 3-methylthiopropyl glucosinolate is sequentially converted to 3-methylsulphinylpropyl, and then to 2-propenyl glucosinolate, by the action of dominant alleles at two loci. RFLP mapping positioned both loci on the same linkage group homologous to the B. napus N19 linkage group. The implication of the results for the genetic manipulation of glucosinolates in Brassica to improve flavour and nutritional properties, and in order to investigate plant-insect interactions, is discussed.