The morphology of the endobiotic and epibiotic stages of Physoderma johnsii Sparrow on Caltha palustris is described. Highly characteristic of the endobiotic stage is the formation of numbers of large, narrowly pyriform cells with a tuft of rhizoids at the broader (distal) end. Early developmental features are not included since germination of the resting spore has not as yet been achieved. Reasons for maintaining this taxon distinct from older ones on Caltha palustris are given.