Propanil (3',4'-dichloropropionanilide), the active ingredient in the herbicide Stam M-4, is possibly the most extensively used herbicide for rice production in the world. Propanil and its metabolites are transported within characteristic ditch ecosystems in the production landscape of northeast Arkansas. Runoff from these ditch ecosystems is further transported to a river or other water body supplied by the Mississippi River Alluvial aquifer. Forty-eight-hour acute toxicity tests with Ceriodaphnia dubia (cladoceran) and Pimphales promelas (fathead minnow) were conducted on stormwater runoff, laboratory synthetic water, and irrigation (ground) water. No effects on survival were observed in this study following 48-h toxicity testing with the stormwater. Survival studies indicated assimilative capacity in irrigation (ground) water as opposed to laboratory synthetic water. Mean 48-h LC50s of C. dubia increased from 2.94 mg/L Stam M-4 in laboratory synthetic water to 8.01 mg/L Stam M-4 in irrigation water. Likewise, P. promelas mean 48-h LC50s increased from 23.76 (laboratory synthetic water) to 33.52 mg/L Stam(R)M-4 (ground water). In 7-d chronic tests, there was an increase in mean LC50s of C. dubia when comparing synthetic water to irrigation water (0.48 to 1.24 mg/L Stam M-4, respectively). P. promelas, however, had less tolerance for Stam M-4 in irrigation water (4.45 mg/L) than in synthetic water (5.93 mg/L) in 7-d chronic toxicity tests. Forty-eight-hour toxicity tests indicate that ground water affords organisms some assimilative capacity that laboratory synthetic water does not. Since herbicides and most other pesticides are manufactured to elicit rapid responses, 48-h toxicity results best describe potential nontarget organism effects in aquatic ecosystems.