ABSTRACTThe development ofCaecomyces communiswas studied with light and fluorescence microscopy.Caecomyces communisis a monocentric fungus which develops to form either a thallus consisting of one or more sporangia attached to a vegetative cell or a mycelioid thallus. During the formation of a vegetative cell with attached sporangia, the posteriorly uniflagellate zoospore encysts and produces a broad germ tube which develops into an incipient vegetative cell. The vegetative cell enlarges faster than the zoospore cyst which develops into an incipient sporangium. A unisporangiate thallus consists of one sporangium attached to a large, vacuolate, vegetative cell. Thallus development may terminate at this unisporangiate stage or further development into a multisporangiate thallus may occur. During multisporangiate thallus formation, the vegetative cell forms one or more outgrowths that become sporangial stalks. Nuclei occur within the vegetative cell and, at maturity, sporangia are formed at the tips of the outgrowths. Often, instead of forming sporangia at the tips of outgrowths produced by the vegetative cell, the outgrowths enlarge into thick filaments. These filaments branch extensively and nuclei occur within the branches. Sporangia are formed at the tips of such filaments which results in a mycelioid thallus. The development and morphology of C.communisandC. equiare compared.