Many efforts have been made to evaluate organophosphorus (OP) toxicity by sensitive biomarkers. Therefore, the aim of this work is to evaluate cholinesterase and paraoxonase (PON1) enzymes activities as diagnostic tools in acute & chronic organophosphorus toxicity with poisoning severity assessment.The present study was conducted on 90 adult men after taking their informed consent, they were divided into three groups; group (I) included thirty patients who were acutely exposed to organophosphorus insecticides (OPI). Group (II) included thirty farm workers chronically exposed to OP. Group (III) included thirty healthy matched volunteers served as control group. The severity of symptoms and signs of acute OP poisoning was graded into mild, moderate and severe grade.  Long term pesticide exposure intensity was estimated depending on mixing, application methods, repair activities and use of personal protective equipment (PPE). Exposure intensity score = (Mix + Apply + Repair) х PPE. Butyrylcholinesterase (BuChE), acetylcholinesterase (AChE) and paraoxonase (PON1) enzymes activities were determined. The result of the current study revealed significant decrease in BuChE, AChE and PON1 in groups I and II when compared to group III. Moreover, BuChE and AChE enzymes were significantly decreased in group I when compared to group II. Significant positive correlation was detected between AChE enzyme activity and both BuChE and PON1 enzymes activities in group I. However, group IIshowed significant positive correlations between BuChE enzyme activity and the activity of each of AChE and PON1. Moreover, there was significant decrease in AChE enzyme activity in severe cases compared to mild and moderate cases in group I. Nevertheless, group II registered significant decrease in BuChE, AChE and PON-1 when Pesticide Exposure Intensity Score is more than 10.