Prickly lettuce is an annual weed that germinates in both the fall and the spring. It is often found in no-till soybeans and winter wheat in Ontario, Canada, as well as along the edges of fields. Field studies were conducted from 2001 to 2004 to estimate crop yield losses, and to characterize the phenology and seed production of prickly lettuce in relation to time of emergence. Prickly lettuce had a large impact on soybean yield, with losses of 60 to 80% at densities of 50 plants m−2 or more. Prickly lettuce density estimated to cause a 10% soybean yield loss varied from 0.2 plants m−2 in 2002 to 1.2 plants m−2 in 2003 and 2004. In winter wheat, prickly lettuce at densities up to 200 plants m−2 caused no detectable yield loss in this study. Plants that emerged in the fall generally were larger, flowered earlier. and produced more seeds than those emerging in spring, but size and fecundity were strongly density-dependent. The number of flowers produced per plant could be estimated from the height of the main stem. Seed production per plant ranged from 2,200 to 67,000 in soybeans, and up to 87,000 in a noncrop area at the edge of the field. Winter wheat harvest interrupted prickly lettuce flowering, and only about 25 to 30% of the plants present in the wheat crop survived harvest and flowered in untreated stubble. These plants produced less than 4,000 seeds per plant. Postharvest control with glyphosate, mowing, or cultivation prevented prickly lettuce seed production in wheat stubble. This study suggests that prickly lettuce populations could build up quickly in continuous no-till soybeans, but rotation with winter wheat and control of plants at the edge of the field would help to limit population growth.