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The ethanolic extract of Polyalthia longifolia var. pendula was fractionated to get a hexane-soluble residue, which led to the isolation of four clerodane diterpenes: (−)-3α,16α-dihydroxycleroda-4(18),13(14)Z-dien-15,16-olide (1), (−)-3β,16α-dihydroxycleroda-4(18), 13(14)Z-dien-15,16-olide (2), (−)-16α-hydroxycleroda-3,13(14)Z-dien-15,16-olide (3) and (−)-16-oxocleroda-3,13(14)E-dien-15-oic acid (4). Diterpene 1 is a new compound, while 2 is reported for first time from this plant. Both 1 and 2 were tested for their growth inhibitory activity on four cancer cell lines in vitro. IC₅₀ values suggest that they are effective as cytotoxic agents.