Four field experiments were conducted in 1983 and 1984 at the Fortuna Research and Development Center, Juana Diaz Puerto Rice to determine the economic threshold of pigweed (Amaranthus dubius); jungle rice (Echinochloa colonum) and horse purslane (Trianthema portulacastrum} and their critical period of competition in drip irrigated tomatoes (Lycopersicon esculentum Mill, var. Duke). Tomato seedlings were 28 days old at transplanting. In the economic threshold study, weed populations were adjusted to 2, 5, 10,20,40 and 80 plants m-2. In the critical period study, mixed weeds were allowed to compete for 0, 12, 24, 36, 48 or 60 days after transplanting and then were kept weed-free until harvest. The economic threshold was found to be 5 plants m-2 for pigweed; 401 plants m-2 for jungle rice; and 20 plants m-2 for horse purslane. This result suggests that pigweed would cause the greatest economic loss to vegetable growers. Pigweed was followed by horse purslane and jungle rice in descending order of causing economic losses. The critical period of weed competition was determined to be 0 to 36 days in 1983 and 0 to 48 days in 1984. ----- Cuatro investigaciones del campo fueron conducidos en 1983 y 1984 en el Fortuna Research and Development Center, Juana Dlaz, Puerto Rico, para determinar los niveles econdmicos de Amaranthus dubius, Echinochloa cotonum, Trianthema portu/acastrum y sus periods crIticos de competicion en tomate [Lvcopersicon esculentum Mill. 'Duke') regado por goteo, AI trasplantar, las plantas de tomate de semillero tertian 28 dias de edad, En el estudio de niveles econ6micos, poblaciones de las malezas eran de 2, 5, 10,20,40 Y80 plantas m-2 • En el estudio de periodo crftico, mezclas de yerbas malas fueron permitidas competir por 0, 12, 24, 36, 48 0 60 dras despuls de transplantar, y entonces, mantenido libre de yerbas malas hasta cosechar, Se encontrd'que el nivel econ~mico era 5 plantas m·2 para A. dubius; 40 plantas m· 2 para E. cotonum: y 20 plantas m-2 para T. portutecestrum. I~I resultado sugiere que A. dubius causaria la major pe'rdida economica a cultivadores de hortalizas. A. dubius fue seguido por T. portutecestrum y entonces E. cotonum en orden descendiente de causar plrdidas econdmicas. Se deterrnind que el periodo cri'iico de cornpeticidn por malezas fue entre 0 y 36 dias en 1983, y entre 0 y 48 d(as en 1984.