The taxonomic significance of morphological characters in fungi of Glomales (Zygomycetes) has been based solely on superficial resemblance. Ontogenetic comparisons among isolates of Scutellospora pellucida and Scutellospora heterogama were used to resolve discrete stages of differentiation in which characters were delimited and ordered hierarchically according to temporal and spatial origin in development. Character concepts were revised, and both species were redescribed. A spore wall, two inner walls, and a germination shield were designated primary characters because they appeared separately and in linear succession. Secondary characters included distinct layers differentiated within each wall. Tertiary characters were qualitative and quantitative properties of each layer. All characters in each developmental stage did not vary in two hosts, in separate experiments, and among five isolates of each species. Stability was attributed to causal epigenetic linkages between stages of differentiation, wherein each new stage depended on differentiation of all characters in the preceding stage. Characters at successively lower hierarchical levels are predicted to specify progressively less inclusive taxa in cladistic analysis. Developmental patterns will improve reinterpretations of phylogenetic relationships and provide a more empirical basis for grouping and ranking of organisms into species and higher taxa at the morphological level. Key words: evolution, morphology, mycorrhizae, taxonomy, VAM fungi.