The septal apparatus of Polyporus biennis comprises a swollen dolipore, a pair of occluding plugs, and perforate parenthesomes differentiated from the wall endoplasmic reticulum. The pore occlusions have an opaque terminal portion and a translucent stem inserted in the pore. The domain between the parenthesome and the dolipore is traversed by microfilaments which may serve to bind the septal apparatus together. Matrix material is present between the parenthesome membranes, producing a lamellar appearance. This unique septum is typical for most basidiomycetes.