A cultivation system has been developed for Penicillium urticae (NRRL 2159A) which yields 'microcycle' conidiation in submerged culture. Spherical growth of conidia was initiated by incubation at 37 °C in a growth-favoring medium. Transfer of these enlarged conidia to a nitrogen-poor medium at 35 °C resulted in synchronous germination and limited outgrowth followed by roughly synchronous conidiogenesis. An ultrastructural study of the germination stage indicated nuclear migration into the emerging germ tube whose new cell wall was an extension of the parent conidium's innermost cell wall layer. Septal formation at the neck of the germ tube followed. The septal pore was filled with particulate material and the septal membranes possessed unusual linear elements in their median hydrophobic zones. The germ tube, which possessed a smooth-surfaced plasma membrane, continued to elongate with periodic septum formation. The parent conidium and later the proximal germ tube showed progressive vacuolation and the cytoplasm became largely occupied by electron-translucent material. In older cells the septal pore was blocked by Woronin bodies. Compared with normal conidial germination this 'microcycle' germination is far more synchronous and the resultant germling is morphologically simpler. In ultrastructural terms, however, germination appears to be identical with that obtained at 28 °C.