The aim of conducted research was the identification of resistance and cross-resistance of Polygonum aviculare, P. convolvulus and P. persicaria to photosystem II inhibiting herbicides (atrazine, simazine, metribuzine, metamitron, linuron, lenacil, bentazone and chloridazone). The research was conducted as monitoring tests. During eight years (2000–2007) seeds of weeds were collected from 243 fields in South-Western Poland. Resistance of biotypes was diagnosed by biological tests (evaluation of phytotoxicity, measurement of fresh plant mass and calculating of resistance index) and fluorescence method. Identified biotypes showed, in most cases, high level of resistance (IR > 6). Most of Polygonum biotypes were resistant to herbicides from triazine group (artazine, simazine, metribuzine and metamitron), lenacil and chloridazone. Resistant biotypes of Polygonum were identified on 15–20% of monitored fields. Participation of resistant biotypes, for all monitored fields, in Polygonum communities did not exceed 40%. On monitored fields also several cases of cross-resistance was determined. Polygonum biotypes were resistant to atrazine and other triazines (simazine, metribuzine and metamitron), atrazine–lenacil and lenacil–chloridazone.