1. The acetylcholinesterase activity of the fruit fly, Drosophila melanogaster, was characterized biochemically. 2. The activity is associated with a glycoprotein which is divided between a detergent-extractable membrane-bound fraction and a soluble fraction. 3. The acetylcholinesterase activity is concentrated in the head of the insect. 4. Through pharmacological methods, greater than 95% of the cholinesterase is judged to be true acetylcholinesterase, and not pseudocholinesterase. 5. As expected for an acetylcholinesterase, the enzyme has a high affinity for acetylthiocholine and is inhibited by excess concentrations of acetylthiocholine. 6. The soluble enzyme is found predominantly as a 7.8 S form; a smaller amount of an ~ 6 S form is also present, and a ≥ 14 S form may exist. 7. The detergent-solubilized acetylcholinesterase has a sedimentation coefficient of 7.5 S in the presence of detergent. 8. The thermal inactivation rates for the soluble and the membrane bound enzymes are markedly different. © 1985.