The insecticide malathion (57% E.C.) was applied at the rate of 712.5 gm active ingredient per feddan on dill, Anethum graveolens L. and coriander, Corianderum sativum L. for controlling aphids infesting these plants. An analytical method, using gas chromatography equipped with flame photometric detector was used for detecting the insecticide residues. A field trial was conducted to determine the rate of dissipation of malathion in dill and coriander plants and in the resulting oil. Residue analysis showed that the initial deposits determined one hour after application were 35.81 & 22.7 ppm in dill and coriander plants, respectively. Rates of dissipation of malathion were 4.72, 51.1, 68.39, 88.41 and 93.49% in dill plants and were 13.61, 43.22, 66.78, 86.26 and 91.85% in coriander plants at 1, 3, 7, 14 and 21 days post treatment, respectively. The pesticide was decayed quite rapidly in and on dill and coriander plants and detectable residues (1.62 and 0.93 ppm) were observed in these plants 28 days after treatment. At harvest 46 days for coriander and 70 days for dill after application malathion was found at average levels of 0.78 mg/kg and 0.54 mg/kg in dill and coriander dry seed, respectively. The volatile oil extracted from the seed by steam distillation process was contaminated with the insecticide at a higher levels than in the seed [about sevenfold in dill oil, 5.21 mg/kg and ninteenfold in coriander oil 10.16 mg/kg]. This means that malathion had tendency to co-distill with the dill and coriander oil throughout steam distillation process.