Limited reports as compared to other insecticides appear in the literature for acetylcholinesterase (AChE) inhibition by diazinon. In the current study, new kinetic parameters of AChE inhibition by diazinon have been investigated. The assay was done with bovine retinal AChE using two different substrate (ASCh) concentrations in the absence and presence of diazinon (0.08-1.28 mM). The optical density was monitored up to 25 min (reaction time) for the assay. New kinetic parameters k'(oms), K'(sms), k(oms), K(sms), K'(asms) and K(asms) ) were calculated from these experimental data.