The phthalate ester insensitive blue-green algae (Synechococcus lividus) were used as a food source to extend the survival of synchronously hatched brine shrimp (Artemia salina) larvae allowing measurement of a reduced toxic response to phthalate esters at late post-hatching stages of development. The maximum acute toxicity due to di-n-butyl phthalate (DNBP) correlated with the expression of a phthalate ester-hydrolyzing enzyme. The purified enzyme was identified as a butyrylcholinesterase due to its rapid inactivation by low. concentrations (10(-7) M) of diisopropyl fluorophosphate and inhibition by physostigmine (IC50 = 6 x 10(-7) M) and tetraisopropylpyrophosphoramide (I-OMPA, IC50 = 5 x 10(-6) M) but not by BW284c5. Apparently competition of the phthalates with the endogenous substrates of the enzyme led to development-dependent toxicity. (C) 2002 Elsevier Science (USA). All rights reserved.