Greenhouse, field, and laboratory experiments were conducted in northern Greece during 1996, 1997, and 1998 to study possible metribuzin resistance in Amaranthus retroflexus (redroot pigweed) and Chenopodium album (common lambsquarters) biotypes found in potato fields. The greenhouse experiments indicated that the suspected resistant biotypes (R) of both species were not controlled by metribuzin applied either pre- or postemergence at rates of 245, 490, 980, and 1,960 g ai ha−1 (the higher rate is eight times greater than the rate recommended for weed control in Solanum tuberosum [potato]). However, susceptible biotypes (S) were completely controlled by 245 g ai ha−1. Also, both R- and S-biotypes of either species were effectively controlled by prometryn applied either pre- or postemergence at 1.5 kg ai ha−1. The field trials confirmed that metribuzin applied either pre- or postemergence at rates of 490, 980, and 1,960 g ai ha−1 gave fair or partial control of the R-biotype of C. album, whereas prometryn applied preemergence at 1.5 kg ai ha−1 gave excellent control of this weed. Chlorophyll fluorescence measurements performed in laboratory experiments indicated that photosynthetic electron transport in metribuzin-incubated leaves detached from plants of the R-biotypes was not affected, but it was inhibited in leaves detached from plants of the S-biotypes. Electron transport was inhibited by prometryn in leaves detached from both S- and R-biotypes of either species. These results show clearly that the biotypes of both species developed resistance to metribuzin, but they were not cross-resistant to prometryn.