A cultivation system has been developed for Penicillium urticae (NRRL 2159A) which yields 'microcycle' conidiation in submerged culture. Spherical growth of spores was initiated by incubation at 37 °C in a growth-favoring medium. Transfer of these enlarged spores to a nitrogen-poor medium at 35 °C resulted in synchronous germination and limited outgrowth followed by roughly synchronous conidiation. An ultrastructural study of the spherical growth stage indicates a significant loss of cell envelope fine structure and a general increase in the number of cellular organelles. Loss of the complex pattern of rodlet s on the spore surface, and the trench-like invaginations of the plasma membrane were most prominent. From an ultrastructural point of view these large spores (about 6 μm in diameter) appeared to be perfectly normal.