Field trials were carried out in 2006 to evaluate the role of aphidophagous syrphids and other natural enemies in the suppression of the lettuce aphid, Nasonovia ribisnigri. The lettuce aphid is the most important arthropod pest of lettuce on California’s Central Coast. Organic growers rely entirely on endemic natural enemies to suppress N. ribisnigri and other aphids in lettuce. Most organic growers provide floral resources to adult syrphid flies in the form of in-field insectary plantings with the intention of increasing predation of aphids by syrphid larvae. We evaluated the impact of natural enemies on N. ribisnigri-infested romaine under three conditions: 1)infested romaine grown under floating row cover to exclude all predators and parasitoids; 2) infested romaine treated each week with spinosad, which kills syrphid larvae but has limited impact on aphids at the rate used; and 3) infested romaine grown in the open without insecticide treatment, as is typical for organic lettuce production in California. In order to confirm the integrity of the floating row cover, a fourth treatment was added that consisted of insect-free transplants that were grown under floating row cover. These four treatments were replicated four times in each trial and the trial was repeated three times. A simpler experiment in which spinosad-treated romaine was compared to untreated romaine was conducted on three certified organic farms. All plots were sampled weekly from transplant to harvest. Data were recorded on densities of aphids, syrphid larvae, and other natural enemies. Our data support the hypothesis that a complex of syrphid fly species is primarily responsible for suppressing aphid populations to below economic levels in organically-grown lettuce on California’s Central Coast, with only limited contribution to aphid mortality from other natural enemies.