Chlorothalonil is a commonly used fungicide in rural and urban environments and can be incidentally introduced into aquatic systems through rainfall runoff or direct overspray and drift from aerial applications. Few studies have been published regarding risks to aquatic organisms exposed to chlorothalonil, so this study was performed to provide a first-order risk characterization for receiving system biota. Definitive laboratory toxicity tests were conducted with aqueous solutions of chlorothalonil and sentinel aquatic organisms (Ceriodaphnia dubia Richard and Pimephales promelas Rafinesque). P. promelas was more sensitive (7-day potency=6.1% mortality/μg/L) than C. dubia (7-day potency=0.94% mortality/μg/L) to chlorothalonil exposures. All mortality of P. promelas and C. dubia resulting from these chlorothalonil exposures occurred within the first 96h and no sublethal effects (i.e., growth or reproduction) were detected under these experimental conditions following 7-day exposures.