The efficiency of two insecticides, fenitrothion (Organophosphorus) and deltamethrin (Pyrethroid) against the 3^rd instar larvae of Culex pipiens were evaluated under the laboratory conditions. LC50 values for fenitrothion and deltamethrin were 0.089 and 0.0016 ppm, respectively.  Also, the effect of the tested insecticides on some biochemical parameters was carried out through evaluation the effect of sublethal concentration of the tested compounds on the activity of acetylcholinesterase, aspartate aminotransferase (GOT), alanine aminotransferase (GPT) and protein content after treatment the 3^rd larval instar for 24 hr. The results indicated that deltamethrin and fenitrothion reduced in the activity of acetylcholinesterase reached to 69.98, 68.0, 73.43 and 65.99% compared to the untreated check with moribund and alive larvae, respectively. As for the transaminases GOT, fenitrothion showed great inhibition on GOT of moribund larvae compared with negligible reduction than the untreated value in alive larvae, showing 97.34 and 16.15% reduction, respectively. On the contrary, deltamethrin caused stimulatory effects on GOT activity in both moribund and alive larvae, reaching 41.15 and 30.21% increase than the normal value, respectively. As for total protein, data clearly showed that both the tested insecticides caused slight alteration in such parameter