The responses of 15 weeds (Agropyron repens, Centaurea cyanus, C. jacea, Chrysanthemum leucanthemum, Cirsium arvense, C. vulgare, Daucus carota, Erigeron canadensis, Euphorbia cyparissias, E. esula, Nasturtium officinale, Neslia paniculata, Plantago major, Silene cucubalus, Vicia angustifolia) to eight herbicides were considered in relation to the level of ploidy, the life form, the nuclear volume, and the interphase chromosome volume, for each species. In general, susceptible weeds had a lower nuclear volume than those showing an intermediate or a resistant response to the herbicides. Interphase chromosome volume did not show any consistent relationship with the degree of susceptibility. Intermediate and resistant plants were mostly perennials, whereas susceptible ones were annuals or biennials.