Red imported fire ants, Solenopsis invicta (Buren) (Hymenoptera: Formicidae), are abundant in many agroecosystems in the southern United States and can affect the abundance of arthropods in these systems. We determined the effects of red imported fire ants on the abundance of aphids, other herbivorous insects, and beneficial arthropods in Alabama tomato (Lycopersicon esculentum) by manipulating the density of red imported fire ants in plots of tomato plants and by sampling fresh market tomato farms for two years (2003 and 2004). In both years of our study, aphid abundance was significantly greater in tomato plots with high densities of fire ants than in plots where fire ant densities were suppressed. Further, the abundance of fire ants was positively correlated with the abundance of aphids on intensely managed tomato farms in both years. These aphids included many species that are the primary vectors of economically-important plant viruses of tomato and other vegetable crops. The positive effect of fire ants on aphid abundance was likely due to facultative fire ant-aphid mutualisms. Other studies have demonstrated that fire ants protect honeydewproducing insects from natural enemies, and we found that fire ants reduced the abundance of beneficial arthropods in the second year of our field experiment. However, red imported fire ants did not significantly reduce the abundance of non-aphid herbivores in either year of our field experiment, suggesting that fire ants are not important biological control agents of these insects in tomato. Fire ants may disrupt biological control of aphids in tomato fields and suppression of fire ants on tomato farms may decrease the abundance of aphids.