Boehmeria siamensis Craib, Shrubs or small trees, simple or branched distally, 1-3 m tall, to 1.5 cm in diam. Dioecious or monoecious. Buds ovoid, 2-5 mm; scales triangular-ovate. Leaves opposite, subequal in size; stipules lanceolate, 7-10 mm; petiole 0.3-1.2 cm; leaf blade light green when dry, narrowly ovate, elliptic, or rarely oblong, thickly papery, secondary veins 3 or 4 each side of midvein, abaxial surface with reticulate vein prominent. Glomerules on axillary, densely congested, spikelike shoots, simple in distal part of twigs or in groups of 2 or more in axils of fallen leaves. Male flowers 4-merous, sessile; perianth lobes elliptic, 1.8-2 mm, densely puberulent, connate to middle. Secondary forests, sunny thickets; 4 China,Laos, Myanmar, Thailand, Vietnam. Two new phenanthroquinolizidine alkaloids, boehmeriasins A and B, were isolated from the aqueous ethanolic extract of Boehmeria siamensis Craib (Urticaceae) by bioassay-guided fractionation.