Technical Abstract: Leafminer (Liriomyza langei Frick) is a major insect pest of many important agricultural crops including lettuce (Lactuca sativa L.) The goals of this study were to evaluate lettuce genotypes for resistance to leafminer and to estimate the heritabilities of three leafminer-resistance traits. Forty-six lettuce genotypes were evaluated in two tests in insect cages. Wild species (Lactuca serriola L., Lactuca saligna L., and Lactuca virosa L.) had significantly fewer leafminer stings than cultivated lettuce (L. sativa) in both tests. PI 509525 (L. saligna) had few leafminer stings and no flies emerged from it. Leaf (leaf and romaine) type of lettuce also showed significantly less stings than the head (crisphead and butterhead) types, while the differences between leaf and romaine lettuces and between crisphead and butterhead types were not significant. Broad-sense heritability for number of stings per unit leaf area was relatively high, averaging 65% over the two tests. Heritabilities for egg-hatching period and flies per plant were 10% and 15%, respectively. Stings per unit leaf area from the two tests were highly correlated (r=0.828), suggesting that the resistance was stable over different plant ages and against different pressures of leafminer. These results suggest that genetic improvement of cultivated lettuce for leafminer resistance is feasible.