An isolate of the purple-brown spored agaric Psilocybe merdaria (Fr.) Ricken, which had been collected on Mull, Scotland, was found to be unstable in its morphological characters when in pure culture. Apart from normal fruit-bodies a series of forms developed, ranging from laterally stipitate to gasteromycetoid ones. The morphological characters of the two extreme forms, before and after several subcultures were made, are described. How these abnormal fruit-bodies might amplify past and present theories on the origin of the Gasteromycetes is considered. The shape of the basidium in this one isolate was also found to be plastic and several deviations from the typical basidium are described. It is suggested that the origin of at least some types of pleurocystidia should be sought in modifications of the basidium. The use of the basidium as a major character in the classification of the Basidiomycetes is not discredited, but a closer relationship than previously expressed between the different groups of the higher fungi is argued. It is suggested that useful information might be obtained from the further study of teratological forms similar to those of P. merdaria whenever they are found in other higher fungi. In the field aberrant forms are not as rare as might be at first supposed.