Interference trials were conducted under greenhouse conditions to determine the effects of mixed densities of smooth pigweed and common purslane on the initial growth of lettuce. Weeds were allowed to interfere with lettuce for 30 days before collecting plant dry matter of each species growing in the containers Densities used were 1, 2, 4, 6 and 8 weeds per container, along with the weed-free control. Monocultures of each weed were established along with the 50:50 proportion within each density, except for 1 plant per container. Interference of monostands of common purslane reduced lettuce dry matter accumulation more than smooth pigweed monostands within each density. Four common purslane plants per container reduced dry matter in as much as 6 or 8 smooth pigweed plants per container. In all cases, within a given density, 50:50 mixture treatments produced more lettuce dry matter than common purslane monocultures.