Axenic cultures ofPeridermiumharknessiiJ.P Moore developed white aerial hyphae, orange callus-like growth, and white mycelium colonies as successive vegetative forms when isolated from explants from infected mature field-grownPinusponderosaDougl. ex. P. Laws & C. Laws. Expiants (small cubes of gall phloem tissue) were collected from March through August 1988 and 1989 in eastern Nebraska and northern and western North Dakota. By 20 days after plating, white hyphae appeared to originate from immature aeciospores. By 45 days, friable orange callus-like growth appeared on many explants from beneath the white hyphae and eventually overgrew the latter. Orange callus colonies consisted mostly of single-celled vesicular cells, which seemed to originate from aeciospore initials. The larger vesicular cells were similar in size to aeciospores, contained orange subcellular globules, and occasionally had surface ornamentation similar to that of aeciospores. After 60 days, white mycelium colonies grew as sectors within and at the edges of many orange callus cultures. The colonies appeared to originate as appendages of thick-walled hyphal strands of orange callus and macroscopically consisted of compact masses of thin-walled hyaline hyphae arising from a stroma. If not transferred to fresh medium, orange callus would frequently convert to white mycelium colonies. No change in the morphology of white mycelium colonies was observed. Only mineral salt media supplemented with peptone, soytone, or a mixture of yeast extract, sucrose, and glucose sustained fungal growth apart from the host tissue. Both orange callus and white mycelium colonies grew over a temperature range of 5–35 °C, and revealed similar staining patterns in 12 of 14 isozymes to those ofPeridermiumharknessiiaeciospores when analyzed with starch gel electrophoresis.