<jats:title>Abstract</jats:title><jats:p>A new, highly active tetrahydro‐<jats:italic>β</jats:italic>‐carboline toxin from the spider <jats:italic>Parawixia bistriata</jats:italic>, the most‐common species of social spider occurring in Brazil, was isolated. The new toxin was identified as 1,2,3,4‐tetrahydro‐6‐hydroxy‐<jats:italic>β</jats:italic>‐carboline (=<jats:italic>N</jats:italic>‐[3‐(2,3,4,9‐tetrahydro‐6‐hydroxy‐1<jats:italic>H</jats:italic>‐pyrido[3,4‐<jats:italic>b</jats:italic>]indol‐1‐yl)propyl]guanidine; <jats:bold>3</jats:bold>). This type of alkaloid, not common among spider toxins, was found to be the most‐potent constituent of the spider's chemical weaponry to kill prey. When <jats:italic>P. bistriata</jats:italic> catch arthropods in their web, they apparently attack their prey in groups of many individuals injecting their venoms. <jats:italic>In vivo</jats:italic> toxicity assays with <jats:bold>3</jats:bold> demonstrated a potent lethal effect to honeybees, giving rise to clear neurotoxic effects (paralysis) before death. The compound's toxicity (<jats:italic>LD</jats:italic><jats:sub>50</jats:sub> value) was determined to be <jats:italic>ca.</jats:italic> 8 ng/g of honeybee. The investigation of the pharmacological properties and neurotoxic actions of <jats:bold>3</jats:bold> may be used in the future for the development of new drugs to be applied for pest control in agriculture.