A total of 150 female O. niloticus weredelivered from a private fish farm and divided into one control group, 2 acute groups (3 and 9 days) and 3 chronic groups (7,14 and 21 days ) . The 2 acute and 3 chronic groups were subjected to ½ and 1/10 LC50 (which was calculated as 0.4 ppm) of thiobencarb respectively . The acute effect of Thiobencarb herbicide on O.niloticus femaleswas manifested in the form of abnormal skin pigmentation in form of darkening ,excessive mucous secretion on the skin and the most prominent sign was skin ulceration, detached scales, fin and tail rot . Fish also showed some nervous manifestation as gasping air and abnormal swimming behaviour. The postmortem findings revealed congestion and hemorrhages in all internal organs especially ovaries, also autolysis in ovaries and dark congested liver. Wherease O. niloticus females in chronic phase showed eye opacity and copied similar clinical signs and postmortem findings as mentioned with the acute in addition to asymmetrical ovaries while the acute was much severe. Females of acute and chronic groups suffered from highly significant drop in both absolute and relative fecundity when compared to control group. Moreover, total protein, sex hormones (estradiol E2 and testosterone T) and AST in chronic   14 and 21 days showed highly significant drop throughout the acute and chronic groups. Oppositely, AST (Aspartate aminotransferase) in acute 3 and 9 days, chronic 7 days and ALT (alanine aminotransferase) in acute 3 and 9 days , chronic 7 ,14 and 21 days showed highly significant increase when compared to control  groups. Finally, owing to the drastic effect of thiobencarb on fish fecundity, it is contraindicated to use it for fish farms, water from agriculture sources using Thiobencarb.