Six populations of Amaranthus quitensis (R1, R3-R7), taken from soybean fields in Córdoba (Argentina), continuously treated with imazethapyr escaped from control with this herbicide. In order to characterize this resistance, whole plant assays were carried out using imazethapyr. Treatments were made at different doses, from 0.5 g a.i.ha(-1) to 60 g a.i.ha(-1), at 200 1 ha(-1) and 200 kPa pressure. Shoot fresh weight was mesured 21 days after treatment. The results were expressed as resistance factor (ED50(R)/ED50(S)). Different degrees of resistance were found as its shown by the respective resistance factors: R1:22.5, R3:6.5, R4:43.1, R5:8.6, R6:4, R7:5. Due to the high variability in the response to imazethapyr treatments shown in some populations, and according to previous investigations on the ALS gene that proved heterozygosity a screening was made, at the recommended dose of imazethapyr. Plants were classified according to their state, dead, alive and affected (fallen but alive). Populations 3, 6 and 7 showed an important heterozygosity. Furthermore, in order to evaluate the existance of a possible cross-resistance, the recommended doses of 4 other imidazolinones (imazametabenz, imazapyr, imazamox, imazaquin) were applied to the plants. All populations were susceptible to both imazamox and imazapyr and showed cross-resistance to imazamethabenz and imazaquin.