Two fish toxins, linifolin a and linifolin b, which are both diterpene orthoesters, have been isolated from Pimelea linifolia (Thymelaeaceae). Pimelea linifolia and Pimelea ligustrina grow along the East coast of Australia. Pimelea species, including P. linifolia and P. ligustrina, have been found to possess anti-cancer activity. In the search for compounds responsible for this activity, two piscicidal compounds have been isolated. Column chromatography of a chloroform extract of P. linifolia on silica gel followed by Sephadex LH20 yielded a mixture of two compounds which was lethal to fish. The two compounds, linifolins a and b, were separated and purified by high pressure liquid chromatography on a reverse phase C18 column. Linifolin a (8×10^-4% of dry weight of plant) is the 12β-acetoxy derivative of linifolin b. Linifolin b (8×10^-4% of plant) was identical with Pimelea factor P2 isolated by Hecker et al. from another Pimelea species. Linifolin b has also been isolated from P. ligustrina (27×10^-4% of plant). Although structurally related to gnidimacrin, which possesses potent anti-leukaemic activity, these two compounds did not show this activity.