The chromosome numbers of 38 Canadian weeds are reported and the significance of some of these numbers is discussed. Counts obtained on material ofIva xanthifoliaL., 2n = 36,Erysimum hieraciifoliumL., 2n = 48, andNepeta catariaL., 2n = 34, differ from those obtained by previous workers. The chromosome numbers of the following species have not previously been reported:Rumex transitoriusRech. f., 2n = 20;R. triangulivalvis(Danser) Rech. f., 2n = 20;R. verticillatusL., 2n = 60.