Onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae), is a worldwide pest of onion, Allium cepa L. In field studies on onion resistance conducted in 2007 and 2008 using 49 cultivars, 11 showed low leaf damage by T. tabaci. In laboratory studies, the 11 cultivars, along with two susceptible checks and four additional cultivars, were evaluated to characterize resistance to T. tabaci and to determine if color and/or light reflectance were associated with resistance to T. tabaci. No-choice tests were performed with adults and the numbers of eggs and larvae were counted on each cultivar after three and 10 d, respectively. In choice tests in which all cultivars were planted together in a circle in a single pot, 100 adults were released and the number of adults on each plant was evaluated 24 h later. The behavioral response of walking T. tabaci adults to plant odors was studied in a glass Y-tube olfactometer. The reflectance spectrum of leaves was measured using a UV-VTS spectrophotometer. Results indicate that resistant cultivars showed an intermediate-high antibiotic effect to T. tabaci and all of them showed a very strong antixenotic effect. There were no significant preferences in the response of walking T. tabaci adults to plant odors. The two susceptible cultivars had the highest values of leaf reflectance for the first (275–375 nm) and second (310–410 nm) theoretical photopigment-system of T. tabaci, and these values were significantly different from most resistant cultivars. These results suggest a strong response of T. tabaci to onion cultivars with higher reflectance in the ultraviolet range (270–400 nm). Overall, these results appear promising in helping to identify categories of resistance to T. tabaci in onions that can be used in breeding programs.