Spider venoms contain a multitude of toxins that target membrane receptors and ion channels. PKTx  a novel  isolated neurotoxic polypeptide from the spider Phoneutria keyserlingi, was partially sequenced and its primary structure shows similarites with alfa-like insecticidal toxins isolated from the venoms of spiders of the genus Phoneutria. In preliminary studies,intracerebral injections of PKTx in mice (3 µg/animal) caused immediate agitation, spastic paralysis, tail elevation, and death after 10 minutes.In order to identify and characterize the effects of PKTx, we firstly investigated its toxicity in mice and house fly. Secondly, we verified  the possible interactions of PKTx with voltage activated sodium channels (Na⁺_v) on mammal and insect synaptosomes. Intrathoracical injections in house flies (Musca domestica) showed active movements of legs and mouth parts, active extension and contraction of the proboscis, paralysis and loss of ability to fly and death of 30% of animals at dose level of 500 ng/animal. These symptoms were dose-dependent. Binding studies with radioiodinated toxins were carried out to determine the sites of interaction of PKTx on cockroach (Periplaneta americana) nerve cord synaptosomes. PKTx (5.10^-7M) partially competed (30%) with ¹²⁵I-Bom IV , an a-like toxin from the scorpion Buthus occitanus mardochei, for the binding on the site 3  of Nav channel present on cockroach nerve cord synaptosomes. In conclusion, our results indicate that PKTx interacts with sodium channels of insect excitable cell membranes, probably as the alpha-like toxins do. Other studies are in development in order to better characterize the interaction of PKTx with insect and mammals  nervous system.