Tx4(6-1) a neurotoxic peptide from the venom of the aggressive South American 'armed' spider Phoneutria nigriventer, has been previously isolated and sequenced. It shows no detectable activity in mice but affects the peripheral nervous system of insects by stimulating glutamate release at the neuromuscular junction. Here we investigate possible interactions of the toxin with voltage-activated sodium channels (Na(v)). We confirm that it is ineffective on mammalian Na(v) channels, and establish that it competes with the alpha-like toxin 125I-Bom IV, for binding on the site 3 of insect Na(v) channel (IC(50) value around 25nM). The physiological consequences of this binding to the insect Na(v) channel are shown by electrophysiology: Tx4(6-1) prolongs evoked axonal action potentials (APs) (<500&mgr;s duration in control). Prolonged 8-10ms or 'plateau' 500-800ms APs accompanied by repetitive firing at 80-150Hz are recorded after 4-8min of toxin action. This modification of evoked activity is due to a slowing down of sodium current inactivation. Effects of Tx4(6-1) on sodium current are compared with those of a typical scorpion alpha-toxin and of some other spider toxins active on insect Na(v) channels. At the end of long voltage pulses, the maintained inward sodium current may represent 50% of the peak current after scorpion alpha-toxin but only about 8-10% after spider toxins. To understand the slight differences in the effects of alpha-scorpion and spider toxins on the insect Na(v) channel, structural studies of toxin-channels interactions would be necessary.