A southern Australian marine sponge, Trachycladus laevispirulifer, yielded the cytotoxic agents trachycladindoles A-G (1-7) as a selection of novel indole-2-carboxylic acids bearing a 2-amino-4,5-dihydroimidazole moiety. The trachycladindoles displayed promising selective cytotoxicity against a panel of human cancer cell lines and their structures were assigned on the basis of detailed spectroscopic analysis. Preliminary structure activity relationship (SAR) investigations by co-metabolite defined structural features key to the trachycladindole pharmacophore, highlighting an unusual bioactive molecular motif deserving of future investigation.