Recently, our group reported the discovery of three new withanolides, physangulidines A–C, from Physalis angulata. In this study, the biological effects of physangulidine A (1), which was the most active and abundant of the three new constituents, are described. It was found that 1 significantly reduces survival in clonogenic assays for two hormone-independent prostate cancer cell lines. Flow cytometry and confocal microscopy studies in DU145 human prostate cancer cells indicated that 1 induces cell cycle arrest in the G2/M phase and causes defective mitosis. It was determined also that 1 produces programed cell death by apoptosis, as evidenced by biochemical markers and distinct changes in cell morphology. These results imply that the antimitotic and proapoptotic effects of 1 may contribute significantly to the biological activities and potential medicinal properties of its plant of origin.