Replicas of concave fractures of freeze-fractured plasma membrane of dormant sporangiospores of Phycomyces show particles of three types, namely, (i) small (5-8 nm); (ii) large homogeneous (30 to 35 nm); (iii) compound (30 to 35 nm). The compound particles appear to be composed of four oblong subparticles (23 nm $\times $ 12 nm). The convex fracture shows only small particles (5 to 8 nm), and also depressions of two types that are complementary to the homogeneous and compound particles on the opposite (concave) face. Heat shocking initiates germination of spores. The compound particles are no longer seen by 6 h after heat-shocking. Aggregates of homogeneous particles appear upon heat-shocking, and these aggregates grow to large arrays by about 12 h. These aggregates and the homogeneous particles disappear by the time the spore germinates, that is, about 20 h after heat-shocking. It therefore appears that both homogeneous and compound particles are related to controlled dormancy and germination of spores. The number of small particles increases on both the fractured faces as the spore germinates. Partial digestion of homogeneous particles by incubation with pronase suggests that these macromolecules may contain protein and are amphipathic in structure. The small particles are apparently unaffected by treatment with pronase. Evidence indicative of pinocytotic activity is seen in germinating spores.