Alterations due to inorganic processes are easily misinterpreted as original stromatoporoid microstructures. Therefore, recognition of alteration phenomena is important. Stictostroma Parks is one of the common stromatoporoid genera occurring in the Middle Devonian of southern Ontario where specimens have been found preserved under conditions of alteration whereby the microskeletal structures are little altered, slightly to greatly altered by recrystallization, or replaced by hydrous silica which has been subjected to various degrees of dehydration. A study of microstructures representing different states of preservation in specimens from different localities, or from place to place in the same thin section suggests the nature of the original morphology of the coenosteum and illustrates the expression of morphology under various kinds of preservation. Microstructures of Stictostroma mamilliferum Galloway & St. Jean, S. excellense (Galloway & St. Jean), S. problematicum (Parks), S. elevatum (Parks), and S. kayi (Parks) are illustrated. Kinds of variation as well as uniformity of microstructure in the 5 species give some understanding of the value of microstructure as a generic or specific character and the stability of such microstructure under different physical and mineralogical conditions.