Triazine-resistant (R) biotypes of common lambsquarters, yellow foxtail, and hairy fleabane were found in the Province of Córdoba (Andalusia, Southern Spain). The former two R biotypes came from atrazine-treated cornfields, whereas the latter came from simazine-treated nontilled olive orchards. The R biotypes of common lambsquarters and yellow foxtail and the R biotype of hairy fleabane survived at doses up to 5 kg ai/ha of soil-applied atrazine or simazine, respectively. Photosynthetic electron transport in R biotypes was unaffected by atrazine and simazine but was inhibited by diuron, as shown by fluorescence induction measurements in whole leaves. In Hill reaction assays, the R biotypes showed high resistance to atrazine and simazine (resistance factors in the range of 350 to 550), medium to high resistance to ametryn, terbumeton, metribuzin, and monolinuron (resistance factors in the range of 80 to 250), slight resistance to diuron and methabenzthiazuron (resistance factors in the range of 1.1 to 15.7), and reverse resistance to swep, ioxynil, and DNOC (resistance factors less than 1). It is concluded that the R biotypes have a chloroplast mode of resistance similar to that previously described for other triazine-resistant weed biotypes.