Extracellular proteases produced by four mycorrhizal fungi growing in pure culture were compared to those produced by their symbionts (mycorrhizal blueberry plant roots). The profiles of the extracellular proteases produced by these isolates in pure culture were different from those produced by the symbionts. This conclusion was further supported by the results from isoelectric focusing study of the protease produced by Sterile White II (H. ericae, UAMH 9264) and its symbiont. No protease band was observed in nonmycorrhizal control plants. Different extracellular proteases were also produced by two different highbush blueberry cultivars infected with the same native isolate, Oidiodendron maius (UAMH 9263). The pH optima for protease activity were different between pure cultured mycorrhizal fungi and their symbionts. These results suggest that extracellular protease production by the symbiont is at least partially host controlled. The results are discussed in relation to ecological aspects of mycorrhizal symbiosis.