Measures of diversity depend on an eclectic taxonomy now being developed from comparisons of morphology, developmental programs, carbohydrate chemistry, fatty acids, and nucleotide sequences in a wide range of arbuscular fungal taxa obtained from living culture collections. Developmental patterns in character origin and transformation are providing clues of intrinsic causation in evolution of diversity. Extrinsic causation is being identified from population-level dynamics, as well as data on species numbers, abundance, composition, and distribution. Detection of species is based solely on sporulation, so that a combination of field sampling and various trap culture methods provide a more comprehensive estimate of fungal community organization. Species distributions rarely correlate with ecological gradients or hypothesized phylogenetic relationships, suggesting that an important causal factor of present-day distributions is dispersal over geologic time. Global distribution of both derived and ancestral species and representation of all genera in most plant root systems further indicate that local diversity has a strong historical component, with ecological processes of subordinate consequence. Ecological dynamics play a crucial role at the local level. They are governed by multilevel diversity among and within organisms of a species assemblage, such as differences in life history traits and heterogeneity of genetic and physiological properties, respectively. Key words: ecology, phylogeny, systematics, vesicular – arbuscular mycorrhizae.