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The exoskeleton lipids of three dangerous pests, Sitobion avenae, Hyalopterus pruni and Brevicoryne brassicae were identified by GC/MS studies. The main components found were triacylglycerols with one hexanoyl group. Fatty acid composition and position of triacylglycerols were determined from mass spectra. There was a trace of triacylglycerol with the E,E-2,4-hexadienoyl group in the extract of Brevicoryne brassicae. A series of hydrocarbons and free fatty acids were identified. Single components of straight chain aldehyde (C 30 ) and alcohol (C 32 ) were detected in the lipids of H. pruni, B. brassicae. Free fatty acids, including these found in the aphid lipids, were subjected to fungi-insect ecological studies. A homologous series of acids were added individually into media used to evaluate the mycelia growth and sporulation tests of the fungi, Beauveria bassiana and Paecilomyces fumosoroseus. The tests were performed in vitro and linear mycelial growth and sporulation of fungi after 14 days were measured. For both fungi, complete inhibition was observed with pentanoic and sorbic (E,E-2,4-hexadienoic) acids at a concentration as low as 0.02% w/v in both tests. Growth stimulation effects were only observed for B. bassiana with tetradecanoic and eicosanoic acid. Inhibitions were noticed for both fungi and the strongest effects were for dodecanoic (B. b.) and eicosanoic acid (P. f.). Beside free fatty acids, no other group of chemical compounds was found in the lipids which could be involved in aphids resistance to entomopathogenic fungi.