Trichophyton tonsurans is a fungus in the family Arthrodermataceae that causes ringworm infection of the scalp. It was first recognized by David Gruby in 1844. Isolates are characterized as the "–" or negative mating type of the Arthroderma vanbreuseghemii complex. This species is thought to be conspecific with Trichophyton equinum, although the latter represents the "+" mating strain of the same biological species Despite their biological conspecificity, clones of the two mating types appear to have undergone evolutionary divergence with isolates of the Trichophyton tonsurans-type consistently associated with Tinea capitis (particularly in children) whereas the Trichophyton equinum-type, as its name implies, is associated with horses as a regular host. Phylogenetic relationships were established in isolates from Northern Brazil, through fingerprinting polymorphic RAPD and M13 markers. There seems to be lower genomic variability in the Trichophyton tonsurans species due to allopatric divergence. Any phenotypic density is likely due to environmental factors, not genetic characteristics of the fungus.