We examined differences in tolerance of Daphnia galeata to a phosphorothioate insecticide, MEP, and the amount of genetic variation for its tolerance among three sampling sites near Takahama-iri bay and three sampling sites near Tsuchiura-iri bay in Lake Kasumigaura and one sampling site in Daizen pond. We performed acute immobility tests for MEP on clone lines that were established from dormant eggs collected in the seven sampling sites. We estimated tolerance values of clonal lines with a toxicant threshold model using results of the acute immobility tests, and calculated genetic variances for the tolerance value of the seven sampling sites. The tolerance values and genetic variances for the tolerance values were compared among the seven sampling sites. Tolerance to MEP did not differ among the seven sampling sites. The genetic variance for the tolerance value marginally differed among the seven sampling sites. Based on these results, we discuss the adaptive responses of D. galeata populations in Lake Kasumigaura and Daizen pond to the exposure of a phosphorothioate insecticide.