The major part of this thesis describes studies on the biosynthesis of the benzoylated drimane sesquiterpene pebrolide and on mycophenolic acid. The location of tritium in pebrolide biosynthesised from [2-T, 2-14C] mevalonate was established by degradation and in particular the 4alpha-oxymethylene group was shown to be derived from the methylene group of mevalonate, in agreement with the specificity of cyclisation found in di- and tri-terpene biosynthesis. A number of degradative sequences for the determining of the level of tritium incorporation into different parts of mycophenolic acid were studied. Application of one of these to material biosynthesised from [2-T, 2-14C] sodium acetate showed that the incorporation of acetate into the phthalide methylene group was in accord with the intermediacy of a tetraketide precursor. A farnesyl analogue of mycophenolic acid, its methyl ether (which were isolated from cultures of P. stoloniferum) and desmethylmycophenolic acid were tested (by radiotracer methods) as precursors, and the final steps in the biosynthesis of mycophenolic acid elucidated. The stereochemistry of a dibydrobenzofuran metabolite of P. brevicomoactum, related to mycophenolic acid, has been established by synthesis from mycophenolic acid. A minor metabolite (brevigellin) of P. brevicompactum was isolated and shown by chemical and spectroscopic means to be a cyclodepsipeptiae containing a previously unreported amino acid residue
