Bracken fern (Pteridium aquilinum) is the fifth most distributed common weed species of the world. Its ecological distribution is very wide, and the plant can grow and spread successfully on many types of soil. The cover of P. aquilinum is--in some cases--remarkable (e.g., in the United Kingdom). Bracken fern contains different poisonous agents: some cyanogen glycosides, factors (agents) of antithiamine character (thermolabile thiaminase and thermostable other compounds) and factors of carcinogenic activity (first of all ptaquiloside). This paper summarises and reviews different toxicological problems and poisonings caused by bracken fern in ruminants (cattle, sheep) and in non-ruminant animals (horses, pigs, rats, mice, etc.). The carcinogenic properties of the norsesquiterpene-type ptaquiloside make bracken fern a potent, living hazard. Recent investigations have shown that ptaquiloside pollution of different soil layers is a distinct possibility. Ptaquiloside may leach from the soil into the drinking water base. This ecotoxicological aspect seems to be the most hazardous phenomenon in relation to P. aquilinum and ptaquiloside. The carcinogenic effect of ptaquiloside is based on its hydrolysis, which leads to the formation of a dienon intermediate. It can produce DNA adducts, which are responsible for inducing carcinoma.