The tetrahydro-b-carboline, callophycin A (1), was isolated from the methanol extract of the red alga Callophycus oppositifolius collected from Pugh Shoal, north east of Truant Island, Northern Territory, Australia. The structure and relative stereochemistry of 1 was determined through extensive NMR analysis. Callophycin A (1) was tested against a panel of mammalian cell lines and found to be generally cytotoxic.