Technical Abstract: The Agricultural Research Service, United States Department of Agriculture announces the release of a breeding line of green leaf lettuce (Lactuca sativa L.) with resistance to leafminers (Liriomyza langei Frick) and lettuce mosaic. The line MU06-857 is similar to cultivar ‘Lolla Rossa’ (‘Lollo Rossa’) except it is larger and has green leaves. It was developed at the United States Agricultural Research Station in Salinas, California by Beiquan Mou and Edward J. Ryder. Leafminer damage occurs when adult flies puncture leaves to feed on plant sap and when females lay eggs within the leaf tissue, leaving “stings” that appear as holes or bumps on the leaves. Resistant varieties could reduce pesticide usage, which would benefit growers, consumers, and the environment. However, lettuce cultivars with a high level of resistance to leafminers are not currently available. Lettuce mosaic virus (LMV) is a serious worldwide disease problem in lettuce. A susceptible plant exhibits systemic symptoms of vein clearing, mottling, leaf recurving, leaf margin frilling, distortion, stunting, and occasional necrosis. It is primarily transmitted by the green peach aphid (Myzus persicae Sulz.), and can cause losses of up to 100%. The breeding line was derived from a cross between a crisphead cultivar ’Salinas 88’ that has mo-1e (mo12) LMV-resistant alleles, and a red leaf cultivar ‘Lolla Rossa’ that showed low leafminer sting density in previous screening experiments. Single plant selections were made by using the pedigree method of breeding from F2 to F6 generation, and MU06-857 is a F7 breeding line. In repeated tests inoculated with LMV, no plants from MU06-857 and resistant control ‘Salinas 88’ developed any symptom, while all plants of the susceptible ‘Glacier’ and ‘Misty Day’ showed typical LMV symptoms. The breeding line had significantly lower leafminer sting density than the resistant control ‘Lolla Rossa’ and commercial cultivars in field trials conducted in Salinas in 2006, 2007, 2008, and 2009. Limited samples of seed are available for distribution to all interested parties for research purposes, including the development and commercialization of new cultivars.