A search was undertaken in the most eastern part of the Himalayan kingdom of Bhutan for the plants which are causing severe losses of yaks due to pyrrolizidine alkaloid poisoning. Two Senecio and three Ligularia species were found on yak pastures at altitudes between 3000 and 4000 m, including one so far underscribed Ligularia species. None was previously known to contain pyrrolizidine alkaloids. Another Senecio species was found between 2500 and 3000 m, an altitude too low for yaks to be kept but significant for other cattle. The search was supported by field chemical tests for the alkaloids and the diagnosis was later confirmed by thin layer chromatography and high pressure liquid chromatography. Two of the Senecio species had exceptionally high concentrations of pyrrolizidine alkaloids of about 0.5 per cent in the dry matter.