Abstract: Homozygotes of the diamondback moth, Plutella xylostella, susceptible or highly resistant to spinosad were obtained by crossing and selection. Biological traits between the spinosad resistant and susceptible strains were compared at a favourable temperature as well as after treatment at low temperatures, in an attempt to determine whether the spinosadresistance was associated with fitness changes and the changes were temperaturerelated. When eggs of both strains were held at 6℃ for 14 days and then moved to 25℃ for subsequent development, percentage of larval survival of the resistant strain was 7.7%, significantly lower than that of the susceptible strain (61.4%). When eggs of both strains were held at 6℃ for 28 days and then moved to 25℃ for subsequent development, percentage of egg survival to hatching was 27% in the resistant strain but 75% in susceptible strain, significantly different between the two strains. When pupae of the resistant strain were held at 5℃ for 7～14 days and then moved to 25℃ for subsequent development, the development duration of pupae became longer and survival of eggs in progeny was reduced in the resistant strain compared to the susceptible strain. However, comparison of performance at 25℃ between the resistant and susceptible strains revealed only minor or no significant differences between them. These results indicate that high resistance to spinosad in the diamondback moth is associated with fitness costs to survive low temperatures.