ABSTRACTThe septal pore apparatus and septum initiation in vegetative hyphae ofAuricularia auricula-judaewere studied by light and electron microscopy for phylogenetic comparisons. Both freeze substitution and conventional chemical fixation methods were employed for electron microscopic studies. Septa developed at the base of the clamp connection and in the main hypha near the clamp where the nuclear divisions had occurred. Septum formation lasted about 4 min. Septal swellings were much larger in chemically fixed than in freeze-substituted hyphae. Fine filaments at the adseptal side of the septal pore cap appeared to be attached to the orifice area of the septal pore and to the cap via intermediate structures. Broader filaments, 7.6 to 9.4 nm in diam, usually connected the plasma membrane bordering the cross wall with endoplasmic reticulum which was continuous with the pore cap. There was a central plate in the septal pore. Ribosomes were absent between the plate and the pore cap. Septum initiation and development involved microvesicles, apparently from the Golgi apparatus, and a third type of filament, 10.6-13.2 nm diam. Septal structure supports the relationship ofAuriculariatoTremellaand functional analysis suggests a general absence of transport of larger organelles between cells, unlike the situation in homobasidiomycetes.