From the lipophilic extract of the marine sponge Acanthella kletha, two new sesquiterpene isothiocyanates have been isolated and characterized as (1R,5R,6R,8S) dec[4.4.0]ane-1,5dimethyl-8-(1'-methylethenyl)-5-isothiocyanate [1] and (1R,5R,6R,8R) dec[4.4.0]ane-1,5dimethyl-8-(1'-methylethenyl)-5-isothiocyanate [2]. The structures of 1 and 2 were deduced from X-ray and spectroscopic (nmr, ir, and ms) data. Sponges of the genus Acanthella have previously been shown to be rich sources of terpenes having various nitrogen-containing functionalities (1,2). On our recent collecting trip to the Great Barrier Reef, Australia, we were able to collect a species from this genus, the bright orange sponge Acanthella kletha Pulitzer-Finali (Axinellidae). The lipophilic extract of our sample of this sponge yielded two new sesquiterpenes 1 and 2 and three previously reported sesquiterpenes 3 (3), 4 (3), and 5 (4). Each of the compounds contained either an isonitrile or an isothiocyanate moiety.