Three new 3-substituted indole diterpenoids (thiersindoles A-C; 1-3) have been isolated from organic extracts of a new Penicillium species (P. thiersii). Their structures, including absolute stereochemistry, were determined by analysis of NMR data and application of Mosher's method. Thiersindoles A-C are biogenetically related to the aflavinines, although the [6,6,5] tricyclic diterpenoid skeleton in compounds 1 and 2 is unprecedented in any of the known members of this class.