A chemical investigation of the digestive glands of Mytilus galloprovincialis from North Adriatic Sea led to the isolation of three novel compounds, oxazinin-1 (1), oxazinin-2 (2) and oxazinin-3 (3). Their structures, including the relative stereochemistry, were established by spectroscopic means including extensive 2D NMR and molecular mechanics calculations. The new compounds were analyzed for cytotoxic activity and oxazinin-1 (1) was shown to inhibit the growth of WEHI 164 and J774 cell lines in vitro.