In Thailand, the shrub Micromelum minuturn (Forst. f.) Seem (syn. M. pubescens Blume) (Rutaceae) (1) is named "Mui-chang" or "Hat-sa-khum," and the stems, fruits, flowers, leaves, and roots are employed medicinally for a variety of indications (2). Previous phytochemical studies have yielded several prenylated coumarins (3-8) and a pyranoquinoline alkaloid (7). Of the coumarins tested biologically, microminutin is weakly cytotoxic (7), and micromelin has antitumor activities (9). From the stem bark, we have now isolated phebalosin, a known prenylated coumarin (10- 12) new to this species, along with a trace ofmicromelin (3,4). The ¹³C-nmr spectrum ofphebalosin, which has not been previously reported (7,9), along with ¹H nmr, high resolution ms, eims, uv, ir, mp, and co-tlc were usehl in identifying the isolated compounds (10- 14). The co-occurrence of the epoxide (phebalosin) with the related carbonyl compound (micropubescin) may be of biogenetic interest (15).Phebalosin was significantly toxic to brine shrimp (16) (LC₅₀ 47 ppm, 95% confidence interval 31-69 ppm) and significantly inhibitory to thedevelopment ofcrown gall tumors on potatodiscs (17)(-64% and -70% in two independent determinations). However, insignificant cytotoxic activity (18) was observed (9KB ED₅₀>20 µg/ml; 9PS ED₅₀ 27 µg/ml), and no activity was observed in the 9ASK astrocytoma reversal assay (19). In the 3PS (P-388) in vivo murine leukemia system (18), phebalosin was inactive in doses up to 25 mg/kg, but insufficient material was available for testing at higher doses.