Biologically active acetylcholinesterase from Drosophila melanogaster was constitutively expressed from a high copy number plasmid in Saccharomyces cerevisiae. Cellular metabolism and plasmid copy number were unaffected by expression. Based on comparative PMSF inhibition, the K-cat value (6430 mol s(-1)) was commensurate with human and Torpedo acetylcholinesterases. A 1.7 knt truncated mRNA transcript was observed on Northern blot analysis.