Summary The structure, development and biology of the fruit body of Fomes levigatus n.sp. are described, with a specific diagnosis. The fruit body is a diageotropic bracket composed of aseptate, un-branched, thick-walled, skeletal hyphae and septate, branched and thin-walled generative hyphae; binding hyphae are absent; the generative hyphae are not clamped. The development of the dissepiments from the generative system is described in detail. The skeletal hyphae of the dissepiments do not become positively geotropic till some time after their origin in the pore field. Radial growth of the bracket is slow with an initial rate of about 1 mm. per 24 hours gradually falling off to zero. In the fruit bodies studied, marginal growth continued for five months : sporing began when the primordia were 2–10 mm. in radius and lasted for seven months. Basidia begin to mature almost immediately after the pores are delimited but only the lower parts of the tubes, for ½I mm. from the pores, are fertile in matured fruit bodies : the upper parts are lined with effete hymenium and stuffed with hyphae derived from the mycelium. Continuous sporing is obtained by the continuous development of new hymenium through the downgrowth of the dissepiments. The fruit bodies cannot survive desiccation. Perennial specimens have not been collected. Comparison is made with the fruit body of Polystictus xanthopus. The Polystictus type is considered the more highly evolved on account of the greater division of labour among the hyphae, the more precise organisation, the continuance of sporing by means of persistent intercalary growth of the hymenium (thus dispensing with long tubes), and the ability to resist desiccation. The distinctive characters of related species, especially of F. senex, F. extensus, and Polyporus gilvus, are enumerated. They have the same systems of hyphae as F. levigatus, but the skeletal hyphae in the flesh of P. gilvus are poorly differentiated, retaining a wide, septate lumen, and are much less numerous than the generative hyphae. Polyporus biogilvus is not nearly related, having three systems of hyphae like P. xanthopus. Fomes velutinosus and F. zelandicus are considered synonyms of F. senex. Two new varieties of F. senex are proposed on the form of the setae, namely var. hamatus and var. bulbosetosus. The terms dimitic and trimitic are proposed for fruit bodies composed of two and three systems of hyphae ; monomitic for those of one system.