Objective:To evaluate the activity of spiromesifen against the most abundant and investigated mosquito species,Culiseta longiareolata Aitken,1954 (Diptera,Culicidae).Methods:Culiseta longiareolata larvae were collected from untreated areas located at Tébessa (Northeast Algeria).A commercial formulation of spiromesifen (Oberon(R) 240 SC) was tested at different concentrations ranging between 238 and 1428 μg/L on newly molted fourth-instar larvae under standard laboratory conditions according to Word Health Organization recommendations.The effects were examined on the mortality,the morphometdc measurements,two biomarkers (catalase and malondialdehyde),and the biochemical composition of larvae,respectively.Results:The compound exhibited insecticidal activity.Moreover,it disturbed growth and several morphological aberrations were observed.It also affected body volume,biomarkers and contents of carbohydrates,lipids and proteins.A marked effect on lipids and malondialdehyde was noted,confirming its primary mode of action on lipid synthesis.Conclusions:Spiromesifen appears less potent than other insecticides tested such as the insect growth disruptors.