Spodoptera exigua, beet army worm caterpillars, and Macrosiphum euphorbiae, potato aphids, fed on Lycopersicon esculentum, tomato plants, in order to ascertain which plant genes were affected by the two different herbivores. Four treatments were performed on the tomato plants: tomato plants fed on by the caterpillars alone, tomato plants fed on by aphids, tomato plants fed on by both caterpillars and aphids, and control treatment with no herbivores. Feeding occurred for 5 days and then the leaves were, immediately, flash frozen in liquid nitrogen. RNA was extracted from the harvested leaf tissue and, with reverse transcription, labeled complementary DNA was formed. The labeled cDNA was hybridized to tomato cDNA microarray chips and scanned with a microarray scanner. We found that caterpillar feeding would normally stimulate tomato defense genes such as protease inhibitors. However when aphids fed simultaneously with caterpillars the plant defense response genes were attenuated.