Teneral females of the twospotted spider mite,Tetranychus urticaeKoch, were utilized in standardized tests to gain information on the nature of the resistance of PI 251303, a tomato line previously shown to be highly resistant to this phytophagous mite. A choice tube technique, previously developed to study behavioral responses of this species to chemical stimuli, and a topical application technique were utilized to determine characteristics of repellency and antibiosis of essential oils of tomato foliage. These oils were obtained by steam distillation and fractionated by GLC. Both repellent and toxic components were found in these fractions. The 2 compounds most toxic toT. urticaewere subjected to mass and infrared spectral analyses which indicated that the compounds are sesquiterpenoids.