SUMMARY: An organism belonging to the Pseudomonas fluorescens species-group has been isolated from fermenting cucumber by enrichment culture, using acetylcholine as the sole carbon source. This organism produces an adaptive cholinesterase. Cells grown in tryptic digest broth have a very low cholinesterase activity. Growth for a few days in the presence of acetylcholine, choline, or Difco nutrient broth produces much higher cholinesterase levels. Cell-free preparations are also active. The enzyme is not identical with either of the main types of animal cholinesterase. It is sensitive to prostigmine, is not inhibited by excess of substrate, and has a unique substrate specificity pattern. The organism also produces two prostigmine-insensitive lipases, splitting triacetin and tributyrin, respectively.