Chemical composition of the Thymus daenensis essential oil and its toxicity was investigated against the first and third instar larvae and adults of Ephestia kuehniella and Plodia interpunctella. Chemical composition of the volatile oil was studied by gas chromatographyâmass spectrometry (GCâMS). Carvacrol (37.0%), thymol (12.8%), β-caryophyllene (7.6%), and geraniol (5.74) were found to be the major constituents of the oil. In fumigant toxicity, the adult stage of E. kuehniella and P. interpunctella (lc50 values of 0.191 and 0.27 μL/L, respectively) were more sensitive to T. daenensis essential oil than first and third instar larvae. In both pests, sensitivity of larvae decreased from first to third instar larvae. However, the first instar larvae of E. kuehniella (lc50=16.30 μL/L) was more sensitive to T. daenensis essential oil than P. interpunctella (lc50=25.32 μL/L) but sensitivity of the third instar larvae of E. kuehniella (lc50=44830 μL/L) was less than P. interpunctella (lc50=3480 μL/L). Repellency of the essential oil in both species was proportional to essential oil concentration. Adults of E. kuehniella with an rc50 value of 0.69 μL/L were more sensitive to repellent effect of the essential oil than adults of P. interpunctella with rc50 value of 6.88 μL/L.