The genetic relatedness between the green and red forms of the two-spotted spider mite, Tetranychus urticae Koch, was analysed using an allozyme marker, MDH, detected by polyacrylamide gel electrophoresis. Cross experiments showed that each of the two forms had form-specific alleles for MDH. Furthermore, there was no overlapping in the alleles for MDH between the field populations of the two forms, suggesting that no gene flow would occur between them in the field.