Pesticides including organophosphates (OPs) and carbamates are used extensively in public health and agriculture to increase crop yields. The abuse of pesticides can have a serious impact on farmer's health. The objective of this study is to study changes in serum cholinesterase activity (BChE) as a biological marker of PO poisoning. Workers in a palm tree oil unit during periods of exposure and non-exposure to pesticides, as well as a control group were selected randomly. The clinical signs reported are those observed in the subjects exposed to organophosphates and anticholinesterase carbamates. The serum enzymatic activity (AES) of cholinesterase was quantified by spectrophotometry. The average serum cholinesterase level during the exposure and non-exposure period was 4225.64 U / l ± 791.10 and 4766.47 ± 873.32 versus 6588.95 U / l ± 1303, respectively, 42 the control level (n = 43) (p <0.001). The average enzymatic activity of workers (n = 19) during non-exposure and exposure periods increased from (78.95%) to non-exposure (36.84%) (Data Not shown) with a significant difference (p = 0.00750). Inhibition of enzymatic activity greater than 40% was observed during the non-exposure phase in 21.05% of cases, and during the exposure period in 26.32% of cases, a bi-annual biological factor necessary for these workers.