As part of the development of an analytical technique for the detection of meridianins and related compounds in biological fluids, a crude extract of the tunicate Aplidium meridianum was analysed using neutral loss tandem mass spectrometry. The 41 u neutral loss- EI(+) mass spectrum showed molecular ions corresponding to two previously undetected alkaloids. We report herein the isolation and structure elucidation of these alkaloids, meridianins F and G.