Acetylcholinesterase (AChE) in the echinoid Lytechinus variegatus has been characterized. Kinetic parameters Vmax, Km, Kss, and b were 2594 ± 1048 nmol ATCh hydrolyzed/min/mg tissue wet weight, 185 ± 11 μM, 308 ± 100 mM, and 0.2, respectively for the substrate ATCh and 17.8 ± 6.87 nmol BTCh hydrolyzed/min/mg tissue wet weight, 654 ± 424 μM, 36 ± 31 mM, and 0.6, respectively for BTCh. Pharmacologic analyses were performed with four inhibitors of cholinesterases, physostigmine, BW284c51, ethopropazine, and iso-OMPA, and yielded IC50 values of 106 ± 4 nM, 718 ± 118 nM, 2.57 ± 0.6 mM, and > 0.0300 M, respectively. Both kinetic and pharmacologic results confirmed the existence of AChE in larval L. variegatus. Dimeric and tetrameric globular forms (determined by velocity sedimentation on sucrose gradients) were present in L. variegatus larvae. Activity of AChE increased significantly as larvae progressed in development from embryos to eight-arm larvae. Acetylcholinesterase activity of F1 larvae derived from sea urchins collected from wild populations and of F1 larvae derived from sea urchins cultured in the laboratory and fed two different diets suggest that the nutritional and/or environmental history of the adult sea urchin affect the developmental progression of AChE activity in the F1 offspring. © 2007 Elsevier Inc. All rights reserved.