Tithoniamarin (1), a new isocoumarin dimer and a new ceramide, named tithoniamide (2a), have been isolated from Tithonia diversifolia , in addition to the known β-sitosterol and β-sitosterol glucopyranoside. The structures were elucidated on the basis of spectroscopic and chemical evidence. Preliminary studies showed that 1 has antifungal and herbicidal activities against Microbotryum violaceum and Chlorella fusca .