Six pesticides were determined by liquid chromatography (LC) with positive ion (PI) atmospheric pressure chemical ionization quadrupole ion-trap tandem mass spectrometry (APCI-MS-MS). Ion fragmentation was studied by MS, MS2 and MS3. Fragmentation of the pesticides produced ions formed by various losses from the side-chains and through heterocyclic ring opening, but without any common fragmentation pathway. Multiple reaction monitoring (MRM) of MS, MS2 and MS3 was used to identify and quantify the pesticides. The samples were extracted with ethyl acetate and dried over anhydrous sodium sulfate. Comparison of the three MS modes showed that MS3 is slightly less sensitive but much more selective. Recoveries from oranges were 72-94% at the limit of quantification (LOQ) level for all MS modes. The LOQs were 0.001-0.3 mg kg(-1) quantifying by means of the MS3 product ion. The method was used to analyse a number of orange samples. This is the first report on using MS3 product ions for quantification of pesticide residues.