Three new merosesquiterpenoids, metachromins U, V, and W (1-3), were isolated from a specimen of the marine sponge Thorecta reticulata collected off Hunter Island, Tasmania, Australia. Structures of the new compounds were elucidated through extensive NMR investigations and comparison with literature values. The cytotoxicities of 1-3 were assessed against a panel of human tumor cell lines (SF-268, H460, MCF-7, and HT-29) and a mammalian cell line (CHO-K1). All compounds were found to have 50% growth inhibition activities in the range 2.1-130 μM, with 2 being the most active (GI50 2.1-10 μM).