A 15 ns molecular dynamics simulation is reported for the complex of mouse acetylcholinesterase (mAChE) and the protein neurotoxin fasciculin-2. As compared to a 15 ns simulation of apo-mAChE, the structural fluctuations of the enzyme are substantially increased in magnitude for the enzyme in the complex. Fluctuations of part of the long omega loop (residues 69-96) are particularly enhanced. This loop forms one wall of the active site, and the enhanced fluctuations lead to additional routes of access to the active site.