In 1985–2002 thirteen weeds resistant to atrazine were selected by a repeated application of triazine herbicides on arable land, in orchards, non-agricultural land and at railways in the Czech Republic. Recently Digitaria sanguinalis biotypes resistant to atrazine have been found at three railway junctions. Long-lasting application of the active ingredient imazapyr at railways caused selection of resistant Kochia scoparia biotypes. High resistance to chlorsulfuron has been discovered in five Apera spica-venti biotypes originating in winter cereals fields. The molecular basis of resistance to atrazine has been identified in the following weeds: Kochia scoparia, Solanum nigrum, Senecio vulgaris, Conyza canadensis, Digitaria sanguinalis, Amaranthus retroflexus and Chenopodium album. The resistance was conferred by a glycine for serine substitution at residue 264 of the D1 protein in all of those weeds. The resistance to imazapyr in Czech Kochia scoparia biotypes was conferred by a mutation at codon 574 of the ALS gene. Analysis of the results of DNA sequencing indicated, that the mutation induced a leucine for tryptophane substitution. There was excellent correspondence between the phenotypic resistance to herbicides of individual plants and the presence of mutations.