The inhibition of acetylcholinesterase, E.C.3.1.1.7, (AChE) activity in the ventral nerve cord of pink shrimp (Penaeus duorarum) by methyl parathion (MPT) and methyl paraoxon (MPO) was investigated. When the animals were exposed to these compounds in water (in vivo), AChE activity of the isolated nerve cord was significantly depressed by MPT only in moribund shrimp after exposure for six hr to 1 microgram/L (96 hr LC50 = 1.9 micrograms/L). Methyl paraoxon (96 hr LC50 = 13.6 micrograms/L) did not depress AChE activity in surviving animals after exposure for 74 hr to 0.98 micrograms/L. Exposure of the excised ventral nerve cord directly (in vitro) resulted in inhibition of 100% after exposure for one hr to 60 micrograms/L MPT, 100% after exposure for one hr to 300 micrograms/L MPO. Implications with regard to the use of AChE activity as a monitor of water pollution by inhibitors of this enzyme are discussed.