The cuticular hydrocarbons of the fourth-instar larvae of Culex pipiens aged one and six days were analysed by gas chromatography and gas chromatography-mass spectrometry. The effect of an insect growth regulator (halofenozide = RH-0345) on the cuticular hydrocarbons was evaluated using newly moulted fourth-instar larvae of C. pipiens, at the lethal concentrations (LC₅₀ = 12.58 and LC₉₀ = 28.58 µ/l). A previous study revealed that RH-0345 is toxic to larvae of C. pipiens by induction of precocious larval moulting. Several cuticular hydrocarbons (CHCs) were identified and many of the other compounds remained unknown. Our data show that the CHC classes consisted of n-alkanes (C23-C29) and differed quantitatively between larval ages. Significant quantitative differences in the distribution of the hydrocarbons of different chain lengths were observed. Amounts of the CHCs of the treated larvae were shifted towards longer-chain alkanes compared to untreated larvae. Typically, treatment with halofenozide decreased significantly the amounts of the major cuticular hydrocarbons of the larvae for the two lethal concentrations, at day 1 and 6. These results provide background data for further studies on the possible role of mosquito cuticular hydrocarbons in chitin synthesis and to better understand the physiological action of halofenozide.