The sublethal toxic potency of malathion in inhibiting acetylcholinesterase activity of brain, muscle, gill and liver tissues of the fish,Tilapia mossambica was studied at 12 h intervals. Maximum in hibition at 36 and 48 h, and complete revival of acetylcholinesterase activity after 72 h was noticed, suggestive of the loss of inhibition of the enzyme activity was probably by suitable (acetylcholine) accumulation.