ABSTRACTThe structure of the concentric granules (CGs) ofAllomyces was studied following both conventional chemical fixation and freeze fracture. The structural features of the CGs are identical when prepared by either method. Analysis by EDAX revealed that the CGs contain phosphorus and perhaps sulfur. In thalli damaged by cutting with a scalpel blade, hyphal compartments adjacent to the damaged compartment frequently did not lose all of their cytoplasm. Rather, cytoplasmic flow was stopped at the septum between the damaged compartment and the adjacent compartment. The pores in these septa became occluded with CGs. Wall material formed between the CGs and the cytoplasm of the hyphal compartment adjacent to the damaged one. These adjacent hyphal compartments were often capable of renewed growth. Ultrastructural studies of the vegetative thallus of Microallomyces dendroideus, a species that lacks hyphal septa, revealed CGs that are structurally identical to those found inAllomycesspecies. Concentric granules that are structurally similar to the ones inAllomycesare present in the thallus ofBlastocladia ramosa.It was concluded that a significant role of CGs inAllomycesis to function as septal pore plugs but other functions for the CGs cannot be ruled out.