<jats:p>The bryozoan Biflustra perfragilis collected in Bass Strait has yielded two novel sulfur-containing isoquinoline alkaloids: 2-methyl-6,7-di(methylthio)isoquinoline-3,5,8 (2H)-trione (1a) and 2-methyl-6-methylthioisoquinoline-3,5,8(2H)trione (1b). A single-crystal X-ray study supports the assignment of (1a).