Fipronil is a phenylpyrazole insecticide increasingly used in applications such as rice culture, turf grass management, and residential pest control, with a high probability to contaminate aquatic environments. As a chiral pesticide, fipronil is released to the environment as a racemic mixture (equal amounts of optical isomers called enantiomers). Enantiomers can have different toxicological and biological activity; however, information on these differences, which is necessary for accurate risk assessment of chiral pesticides, is limited. Here we examine the acute toxicity of fipronil enantiomers, the racemate, and its photoproduct (desulfinyl fipronil) to Ceriodaphnia dubia. The 48-h median lethal concentration (LC50) values based on measured concentrations of each compound indicate the (+) enantiomer (LC50 = 10.3 +/- 1.1 microg/L, mean +/- standard error [SE]) was significantly more toxic to C. dubia than either the (-) enantiomer (LC50 = 31.9 +/- 2.2 microg/L) or racemate (LC50 = 17.7 +/- 1.3 microg/L). To account for any potential loss of fipronil through photolysis, tests were performed under light (fluorescent) and dark exposure conditions, and no significant differences in toxicity were observed. Desulfinyl fipronil, the major photodegradation product, which is not chiral, was detected at < 1% of each parent compound in test solutions after 48 h. Separate toxicity tests with desulfinyl fipronil found a > 20-fold higher LC50 (355 +/- 9.3 microg/L) compared to the fipronil racemate, suggesting lower adverse effects to C. dubia as a result of fipronil photolysis. The present results suggest selection of the (-) enantiomer in fipronil production for lower impacts to C. dubia; however, the consistency and relevancy of fipronil's enantiomer-specific activity at both acute and chronic levels of concern to additional target and nontarget species needs further consideration.