Cell-bound cholinesterase enzyme activity is reported for the first time in the mycelium of Trichoderma hazianum. This enzyme hydrolyzes both the acetylcholine and the butyryl thiocholine esters. The K-m and V-max for choline ester are 0.69 mM and 1.0 nmol acid released min(-1) mu g(-1) protein. However, the thiocholine ester has a K-m value of 2.2 mM and V-max value of 3.33 nmol product formed min.(-1) mu g(-1) protein. The enzyme is inhibited by eserine, a true classical cholinesterase inhibitor.