A significant amount of the total protein in the spore sacs of the microsporidian Thelohania sp. consisted of the cytoskeletal elements, cytokeratin intermediate filaments, and the desmosomal analogues. The cytokeratin and desmosomal analogues were organized as cage envelopes surrounding the spores within the spore sac stage. Thelohania sp. parasitizes the skeletal muscle of Callinectes sapidus, a crustacean that does not appear to have cytokeratins or desmosomes. Immunoprobe data indicate Thelohania sp. has a 240 kDa desmoplakin protein and 48, 51, 54 and 56 kDa cytokeratin polypeptides responsive to antibodies developed against bovine cytoskeletal counterparts. The cytoskeletal envelopes within the Thelohania sp. spore sac stage appear to enhance the stability and viability of the spores.