An Australian marine sponge, Spongia hispida, has been found to contain a new sesquiterpene/quinone identified by detailed spectroscopic analysis and chemical derivatization as the antibiotic 5-epi-isospongiaquinone [3]. The complete stereostructure for 3 was determined by detailed spectroscopic analysis and chemical correlation with the known marine natural product isospongiaquinone [2]. Co-occurring with 3 was an ethylated analogue, 5-epi-homoisospongiaquinone [4], which was speculated to be an artifact of the isolation process. A revised structure 15 for the known marine metabolite, smenorthoquinone [13], is also presented.