Manzamenone G (1), a novel dimeric fatty-acid derivative with a new carbon-skeleton, was isolated from the Okinawan marine sponge Plakortis sp. together with manzamenone H (2), a new tyramine-containing manzamenone congener, and their structures elucidated on the basis of spectral and chemical means. Marine sponges of the genus Plakortis have proven to afford a variety of unique peroxy aliphatic acids and esters. During our studies on bioactive substances from Okinawan marine organisms, we have investigated extracts of the Plakortis sponges and isolated plakotenin, a cytotoxic metabolite, and manzamenones A - F, unique fatty acid derivatives. Further examination of the constituents of the same Plakortis sponges has now resulted in the isolation of manzamenones G (1) and H (2), two new dimeric fatty acid derivatives, possessing bicyclo[4.4.0]decane and bicyclo[4.3.0]nonane skeleton, respectively. The carbon-framework of manzamenone G (1) is hitherto unknown. Here we describe the isolation and structure elucidation of 1 and 2.