Fipronil is a phenylpyrazole pesticide that has greatly increased in popularity in recent years. As a chiral molecule, fipronil is released into the environment as a 1:1 mixture (called a racemate) of its two enantiomers. Previous toxicity work has indicated that the enantiomers of fipronil exhibit significantly different levels of acute toxicity to the nontarget organism Ceriodaphnia dubia. In this work we examine the chronic effects of the pure enantiomers and racemate on the survival, development, mobility, and reproduction of C. dubia adults and the survival and mobility of their offspring. Based on 8-day trials, the (+) enantiomer of fipronil showed a significantly greater reduction in the number of offspring (LOEC = 2 microg/L) than either the racemate (LOEC = 15 microg/L) or the (-) enantiomer (LOEC = 30 microg/L). The (+) enantiomer was also shown to be significantly more toxic to neonates born during the course of the experiment (LC50 24 = 18.1 microg/L, LC50 48 = 10.3 microg/L) than the racemate (LC50 24 = 33.3 microg/L, LC50 48 = 30.3 microg/L), but only after 48 h. Both the (+) enantiomer and the racemate were significantly more toxic to C. dubia than the (-) enantiomer (LC50 24 = 65.2 microg/L, LC50 48 = 50.1 microg/L) at both time points. Qualitative mobility data followed a similar trend, with the (+) enantiomer causing greater impairment in mobility at low concentrations. These data imply that the (-) enantiomer has less impact on the reproductive success of C. dubia than either the (+) enantiomer or the racemate. Enantiomerically pure or enriched formulations of (-) fipronil may reduce impacts to the nontarget organism C. dubia.