Five flavonoids, acacetin ( 1 ), secundiflorol I ( 2 ), mucronulatol ( 3 ), isomucronulatol ( 4 ), and isovestitol ( 5 ) were isolated, with the fractionation being guided by the brine shrimp lethality test (BST), from the ethanolic extract of the whole plant of Robinia pseudoacacia (Fabaceae). The structures of 1 - 5 were identified by spectral analyses. Compounds 2 - 5 are, for the first time, reported from this species. Corrections have been made for the previous literature assignments of the 13 C NMR resonances of compounds 1 - 3 . Bioactivities in BST and cytotoxicities against a panel of six solid human tumor cell lines were determined, and 1 was significantly cytotoxic in the prostate cell line (PC-3).