Acetylcholinesterase (AChE, EC 3117) was extracted from Pieris brassicae heads, by successive homogenizations yielding low-salt soluble (LSS), detergent soluble (DS) and high-salt soluble (HSS) fractions. In all three extracts, two distinct AChE forms were observed, sedimenting at 7.3 and 6.5 S in the presence of Triton X-100. They accounted respectively for 20-40 and 60-80% of the total AChE activity. The 7.3 S component does not interact with Triton X-100 nor with sodium deoxycholate as indicated by sedimentation and electrophoretic migrations. Thus 7.3 S form is therefore considered as a hydrophilic component. The 6.5 S form binds detergent micelles and it aggregates in the absence of Triton X-100. Its electrophoretic mobility is increased in the presence of deoxycholate. This 6.5 S form is therefore a hydrophobic species. These two components have similar substrate and inhibitor specificities and probably correspond to different cellular locations of a single enzyme species. In contrast with vertebrate hydrophobic forms of AChE, the hydrophobic variant of Pieris AChE is not converted into the hydrophilic component by mild pronase treatment. We did not observe any additional form of AChE solubilized at high ionic strength, even in the presence of EDTA. Thus Pieris heads do not apparently contain collagen-tailed AChE molecules, similar to the asymmetric forms described for vertebrate AChE.