Clays are widely used as agricultural chemical carriers, but often, little thought is given to their use as biological carriers. Water activity is of the utmost importance in biological carriers. Clays have great ability to mediate water activity and are often the most important factor in a soil moisture profile. Commercial clays of relatively wide mineral composition are available and can be modified to meet the needs of a particular microbe. Due primarily to their large surface area, clays can act as a desiccant providing good storage stability for dried or biologically inactive formulations. U.S. patents 3,168,796 and 5,695,541 describe using a dried clay/bacterial powder for legume inoculants. U.S. patent 5,849,284 describes a dry clay/nematode formulation. At higher moistures, clay can be used as a relative humidity moderator for semi-active microbial formulations due to surface area and pore size distribution. U.S. patent 5,042,427 describes storage of entomopathogenic nematodes in a clay mediated 60% relative humidity environment. At still higher moistures, due to an absorbent clay's surface area, pore size distribution and total porosity, water activity can be controlled to provide a moist environment for biologically active formulations. U.S. patent 5,288,296 describes growing and delivering inoculants in this manner. At still higher levels of water, clay can act as a substrate for growth. U.S. patent 4,327,181 uses clay as a support for ectomycorrhizae and U.S. patent 4,620,929 uses clay as a fixed growth support in filtration. Microbes colonize the clay surface in a fixed column to remove proteins and nitrates from water. Finally, the clay can be used merely as a granular carrier. U.S. patent 5,358,863 describes granular carriers for bacteria, fungi and nematodes. The organism is formulated in a liquid and applied to the granule. This paper discusses the theory and practice of using clays as biological carriers.