Extracellular proteins play crucial roles in the interaction between mycorrhizal fungi and their environment. Computational prediction and experimental detection allowed identification of 869 proteins constituting the exoproteome of Hebeloma cylindrosporum. Small secreted proteins (SSPs) and carbohydrate-active enzymes (CAZymes) were the two major classes of extracellular proteins. Twenty-eight per cent of the SSPs were secreted by free-living mycelia and five of the 10 most abundant extracellular proteins were SSPs. By contrast, 63-75% of enzymes involved in nutrient acquisition were secreted. A total of 150 extracellular protein-coding genes were differentially expressed between mycorrhizas and free-living mycelia. SSPs were the most affected. External environmental conditions also affected expression of 199 exoproteome genes in mycorrhizas. SSPs displayed different patterns of regulation in response to presence of a host plant or other environmental signals. Several of the genes most overexpressed in the presence of organic matter encoded oxidoreductases. Hebeloma cylindrosporum has not fully lost its ancestral saprotrophic capacities but rather adapted them not to harm its hosts and to use soil organic nitrogen. The complex and divergent patterns of regulation of SSPs in response to a symbiotic partner and/or organic matter suggest various roles in the biology of mycorrhizal fungi.