Synthetic pyrethroids (SPs) are widely used in both agricultural and urban regions for insect control.Unlike many other pesticides, SPs are chiral compounds consisting of stereoisomers. However,occurrence of isomer selectivity in environmental processes is poorly understood for SPs. Weevaluated isomer selectivity in toxicity of cypermethrin (CP) to
Ceriodaphnia dubia and in itsbiodegradation by microbial isolates and in sediment. Among the eight enantiomers, two enantiomers(
1R-cis-S and
1R-trans-S) were found to be toxic to
C. dubia. Bacteria strains isolated from sedimentselectively degraded CP diastereomers and enantiomers. The trans diastereomers were preferentiallydegraded over the cis diastereomers. Of the two active enantiomers,
1R-cis-S was degraded slower,whereas
1R-trans-S was degraded faster than the other stereoisomers. Similar isomer selectivitywas observed during CP degradation in whole sediment. Since ecotoxicity is likely caused only bythe biologically active enantiomers, knowledge on isomer selectivity may improve our understandingof the ecological risks of CP and analogous SPs.Keywords: Isomer selectivity; enantioselectivity; cypermethrin; synthetic pyrethroids; biodegradation.