The nonionic surfactants Tween 20 [oxysorbic (20 POE) polyoxyethylene sorbitan monolaurate] and Tween 80 [oxysorbic (20 POE) polyoxyethylene sorbitan monooleate] at concentrations of 0.05, 0.1, 0.2, and 0.4% v/v stimulated germination of barnyardgrass [Echinochloa crus-galli (L.) Beauv. # ECHCG] seeds in petri dishes. Stimulation occurred under both 30/20 and 25/15 C diurnal temperature cycles whether or not light was supplied during the 8-h high-temperature portion of the cycle. Barnyardgrass responded most to treatment under environmental conditions in which untreated seeds germinated the least. A 0.1% surfactant concentration often stimulated germination of weed seeds as effectively as did higher concentrations. Germination of redroot pigweed [Amaranthus retroflexus L. # AMARE] seeds was inhibited by Tween 80 in the light under both the low- and high-temperature regimes and by Tween 20 at low temperature in the light. Germination of tumble pigweed (Amaranthus albus L. # AMAAL) seeds was inhibited by both surfactants in the high-temperature regime whether or not light was supplied. Common purslane (Portulaca oleracea L. # POROL) seeds were insensitive to treatment.