Senecio or groundsel species (Senecio spp.) and houndstongue (Cynoglossum officinale) contain highly toxic pyrrolizidine alkaloids. These alkaloids are potent liver toxins that cause wasting and photosensitization. Senecios and houndstongue occur on many western U.S. rangelands. Only seven of more than 112 senecio species are known to be toxic so correct identification is essential. Managing rangelands so that plant communities are in good condi- tion and adequate forage is available is crucial to reduce losses to senecio. Generally, senecios are not very palatable, and are avoided by grazing livestock if other forage is available. Drought stress and overgrazing can increase populations of threadlleaf groundsel, as the plant is an aggressive invader. Drought is an especially dangerous time because other forage may be lacking and the toxic alkaloid concentration in senecio plants increases during drought, so grazing animals may ingest higher quantities of more toxic forage. Senecio species are also most toxic when plants are reproducing, thus avoiding pastures when these plants are in bud, flower, or seed is prudent. Proper grazing management must consider stocking rates, as excessive stocking may increase the amount of toxic plant consumed when alternative forages become limited. Excessive stocking may lead to degradation of the desirable plant community allowing senecio species to increase. Herbicidal control may alleviate some problems if incorporated into an overall management program