AbstractPlants ofAmaranthus cruentusandAmaranthus hybridusresistant to atrazine and cyanazine were found in maize fields in north‐eastern Spain. Both resistant foiotypes survived doses of 5 kg ha−1of atrazine and 2–4 kg ha−1of cyanazine but were controlled by lower doses of bentazone and pyridate than were susceptible biotypes. Such a negative cross‐resistance was not found for chloroacetamides and MCPA. Chlorophyll fluorescence studies revealed that atrazine, bentazone, cyanazine and pyridate (10 mg litre−1) caused inhibition of photosynlhetic electron transport in susceptible leaves, while in resistant plants, atrazine and cyanazine had no effect. Conversely, bentazone and pyridate inhibited photosynthesis to a greater extent in resistant than in susceptible biotypes. Isolated chloroplast membranes from resistant biotypes showed resistance factors of 366 and 501 to atrazine and 39 and 60 to cyanazine forA. hybridusandA. cruentus, respectively. Bentazone and pyridate were found to be more effective in chloropiasts of the resistant biotypes than those of the susceptible plants. It is suggested that enhanced susceptibility to bentazone and pyridate in triazine‐resistantA. cruentusandA. hybridusbiotypes may be associated with the alteration of the D‐I polypeptide subunit of photosystem II, as found in triazine