Six norterpenes including negombatoperoxides A and B (4 and 5), the inseparable epimers negombatoperoxides C and D (6 and 7), negombatodiol (8), and negombatolactone (9), in combination with three known compounds, (+)-nuapapuin B (1), (+)-nuapapuin B methyl ester (2), and (+)-aikupikoxide C (3), were isolated from the Formosan marine sponge Negombata corticata. In addition, 6,6-dimethylundecane-2,5,10-trione (10) was isolated for the first time from a natural source. Their structures, including relative configurations, were elucidated on the basis of interpretation of spectroscopic data and by the application of the empirical rule established by Capon and MacLeod. The absolute configurations of 8 and 9 were established by the application of Mosher's method and comparison of CD data with known lactones, respectively. Cytotoxicity of these isolates against human breast carcinoma, human liver carcinoma, and human lung carcinoma cell lines was evaluated.