SummaryResistance of weeds to triazine herbicides has been recorded in many countries. The extent of the problem in South Africa is uncertain. In a pilot study, the atrazine resistance ofAmaranthus hybridusL. (smooth pigweed) was investigated. Suspected resistant (R) and susceptible (S) biotypes were treated with commercially formulated atrazine. After post‐and pre‐emergence applications under tunnel conditions, it was found that the suspected R biotype plants were not affected at herbicide dosages of belween 1.25 and 25.0 kg a.i. ha‐1, i.e. up to 20 times gieater than the lowest recommended dosage. However, the S biotype plants were killed by the lowest dosage. In the fieid, mortalities in the R biolype were not observed after post‐emergence applications of 1.25‐10.00 kg a.i. ha‐1. In contrast, all S biotype plants were killed. In tunnel experiments, the R biotype was also found to be resistant to cyanazine and cyanazine+atrazine, while slight tolerance to linuron was observed. All these treatments resulted in 100% mortality of the S biotype. Although S biotype seeds oiA. hybriduswere found to germinate slightly sooner under controlled conditions than R biotype seeds, preliminary results suggest that there are no major differences. Indications are that, although the growth of the S biotype may be greater than that of the R biotype, the competitive effect of the two biotypes on crop seedlings may well