Three new guaiane-type sesquiterpene alkaloids, muscicapine A (1), muscicapine B (2), and muscicapine C (3) were isolated from the roots of Croton muscicapa. The structures were established by analysis of spectroscopic data, mainly 1D and 2D NMR and MS. This is the first report of a new class of guaiane-type sesquiterpene alkaloids. ©2005 Sociedade Brasileira de Química.