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Sublethal effects of cholinesterase (ChE) activity of mudskippers (Periophthalmus novaeguineaensis) were evaluated in a static, renewable system using organophosphate insecticide, chlorpyrifos. A significant negative relationship between body ChE activity and total length was found (r 2= 0.353; p< 0.0001), with larger fish exhibiting lower ChE activities. Due to the effect of fish size on ChE activity, fish of similar size were used for all subsequent experiments. After 96 h exposures, mortality was highest at nominal chlorpyrifos concentrations of 128 μg L-1 and 256 μg L-1. The inhibition of ChE was dose dependent and ChE activity was greatly reduced after 96 h of exposure of chlorpyrifos at sublethal concentrations. The lowest concentration that significantly inhibited ChE activity after 96 h was 4 μg L-1 and concentrations higher than 100 μg L-1 caused more than 75% inhibition. The IC50 at 96 h was approximately …