Alloxydim is an herbicide marketed as the E-isomer applied at early stages of grass weeds. In the field, sunlight and temperature can induce the isomerization reaction of E-alloxydim to form the mixture of E and Z isomers. A bioassay has been performed to compare the herbicidal activity of E-alloxydim and its corresponding E/Z isomeric mixture in wheat plants. The IC50 values calculated from the root lengths of wheat for E-alloxydim and alloxydim mixture were 0.37 and 0.70 mg L-1, respectively. The Z isomer of alloxydim in the isomeric mixture has no phytotoxic effect on the wheat germination whereas E-alloxydim inhibited the seed germination at low concentrations (from 0.0 to 4.0 mg L-1).