Fish brain acetylcholinesterase is inhibited in vlvo by organic phosphorus insecticides at concentrations of 0.1 rag/1 and lower. The degree of inhibition is a function of the concentration of the insecticide, the extent of exposure, specific chemical nature of the inhibiting substance, and fish species. Species used were largemouth bass (Micropterus salmoides), bluegill (Lepomis macrochirus), golden shiner (Notemigonus crysoleucas), goldfish (Carassius auratus), and fathead minnow (Pimephales promelas). Fish removed from exposure to acetylcholinesterase-inhibiting insecticides, prior to reaching lethal limits, demonstrate the capacity for regeneration of this enzyme. The time required to regenerate the brain acetylcholinesterase to normal levels depends on the extent of the initial inhibition, the specific nature of the compound producing the inhibition, and the particular species. The curve of brain enzyme inactivation and recovery, as defined by percentage changes in normal activity, describes a physiological brain cholinesterase characteristic of the fish species. The period necessary to regenerate to normal activity may be longer than 30 days. By utilization of suitable fish species it is possible to demonstrate the presence of organic phosphorus insecticides in water at concentrations of 0.1 rag/1 with exposure periods of 12 to 24 hours. Some compounds produce marked inhibition of brain enzyme activity in 6 hours. At 0.01 mg/l Guthion produces comparable in vivo inhibition of brain acetylcholinesterase in 12 hours' exposure in several species.

vas and the IToods Hole Oceanographic Institution * Marine installations of various kinds necessitate exposure of construction materials under sea water. Data dealing with the amount of fouling accumulated by such materials are not abundant. Information which might be of aid to the scientist seeking the most favorable material upon which to collect sedentary organisms for study is also scanty. The present study was undertaken to determine the effect

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