In the course of our investigation of the biologically active metabolites of marine invertebrates, we found that the methanol extracts of a gorgonian octocoral of the family Acanthogorgiidae collected at Sukumo Bay, Kochi Prefecture, showed marked lethalty to brine shrimp, Artemia salina L., and insect growth inhibitory activity against silkworm, Bombyx mori L. Bioassay-guided fractionation of the extract led to isolation of five 9,10-secosterols 1-5 and two diterpenoids 6 and 7, which are responsible to the observed activity, together with three furan derivatives 8-10. Their structures have been characterized by extensive 2D NMR studies, including scrutiny of J-resolved spectra, EIMS analysis, and chemical methjods. Biological activities of isolated compounds against brine shrimp and silkworm were evaluated. Secosteroids 1-5 were significantly toxic to brine shrimp and their LD_<50>-values ranged from 1.0 to around 10ppm. Diterpenoids 6 and 7 were active to both of tested animals but furan derivatives 8-10 were inactive to both of them.