In the study of bioactive substances in Illicium plants, the methanol extract of I. merrillianum showed brine shrimp lethality test (BST) activity at 200μg/ml. Bioassay-guided fractionation of the BST active fractions resulted in the isolation of 4-O-methyleudesm-11-en-4α-ol, eudesmol-11-en-4α-ol and (-)-hinokinin as potent BST active compounds. On the other hand, four new highly oxygenated seco-prezizaane-type sesquiterpenes, merrilliortholactone (1), 2 α-hydroxycycloparvifloralone (2), 2 α-hydroxycycloparviflorolide (3), and 2 α-hydroxyanisatin (4) were isolated from the BST-inactive polar fractions. The structures of new compounds were elucidated by extensive analyses of spectral data. Furthermore, the absolute configuration of 3 was established by the modified Mosher's method. Compounds 1-4 showed neither BST activity at 100mg/ml nor neurite outgrowth-promoting activity.