Culture extracts from fermentations of the agaric Macrocystidia cucumis (Pers. ex Fr.) Heim have previously been shown to exhibit antimicrobial activity and strong cytotoxic effects. We now report on the isolation of the active principles from mycelial cultures of M. cucumis, which were identified as a series of biosynthetically related triquinane-type sesquiterpenoids. Arthrosporone (1) and eight new triquinane-type sesquiterpenoids were isolated. The cucumins A–D (2, 4–6) are highly unsaturated hirsutane derivatives, whereas the cucumins E–G (7–9) represent a new type of linear triquinanes. Cucumin H (10) is a new member of the ceratopicane group. The absolute configuration of cucumin F (8) was assigned by 1H-NMR analysis of the corresponding Mosher esters. Two further metabolites were identified as cyclo(phenylalanylprolyl) (12) and cyclo(leucylprolyl) (13). Cucumin A (2) exhibits antibacterial and cytotoxic activities.