A cultivation system has been developed for Penicillium urticae which yields 'microcycle' conidiation in submerged culture. Spherical growth of spores was initiated by incubation at 37 degrees C in a growth-favoring medium. Transfer of these enlarged spores to a nitrogen-poor medium at 35 degrees C results in synchronous germination and limited outgrowth followed by roughly synchronous conidiation. A study of the conidiation stage showed that a phialide and an immature conidium began to form at the tip of all germ tubes 18 h after the temperature shift. By 24 h additional phialides commonly appeared as a branch near the tip of the germ tube and the more mature conidia exhibited increasing refractility. The earliest ultrastructural signs of conidiation were various round invaginations in the plasma membrane and a thickening and rounding of the new spore wall which appeared as an inner extension of the phialide cell wall. Upon segregation of the conidium from the phialide cell by conidial wall formation, 'trench-like' invaginations gradually appeared in the plasma membrane and a disorganized rodlet pattern was formed on the outer surface of the maturing conidial wall. Continued maturation involved the formation of chains of conidia and phialide senescence which was characterized by a general degradation of intracellular structure. A comparison with standard surface and submerged culture conidiation indicated that 'microcycle' conidiation, while less prolific, was essentially identical.