Technical Abstract: 'Mason' is a large-seeded yellow-cotyledon lentil (Lens culinaris Medik.) developed by the USDA-ARS in cooperation with the Washington Agricultural Research Center (Pullman, WA) and the Idaho Agricultural Experiment Station (Moscow, ID) and released in 1997. Mason was released based on its' large seed size, absence of seed coat mottling and high yields. Mason originated from the cross 'Laird'/'Precoz' made in 1982. Laird (1) is a large seeded cultivar developed in Canada and Precoz (2) is a small-seeded early-maturing cultivar from Argentina. The cross and subsequent selection was made to combine large seed size and high biomass production of Laird with earliness and high yields of Precoz. Compared to Brewer, Mason averaged 1 cm taller, flowered an average of one-day later and matured one-day earlier. Plants of Mason are strongly branched at the base, which imparts a bushy structure that enables the canopy to remain somewhat erect during the growing season. Mason has uniform large seeds (100 seeds weigh an average of 7.0 g vs. 6.0 g for Brewer). The large seed size and lack of seedcoat mottling of Mason are features that should appeal to markets in the U.S. and internationally.