Investigation of the chemical constituents from the roots of Melochia chamaedrys (Sterculiaceae) resulted in the isolation of two quinolinone alkaloids: a new alkaloid, named chamaedrone (1), and a known alkaloid, antidesmone (2). Their structures and stereochemistry were elucidated on the basis of 2D NMR and X-ray crystallographic analysis. Furthermore, alkaloids 1 and 2, and the synthetic analogues 3-6 obtained from 2, were screened for antimicrobial activities.