Thyristors are a class of nonlinear electronic device that exhibit bistable resistance - that is, they can be switched between two different conductance states(1). Thyristors are widely used as inverters ( direct to alternating current converters) and for the smooth control of power in a variety of applications such as motors and refrigerators. Materials and structures that exhibit nonlinear resistance of this sort are not only useful for practical applications: they also provide systems for exploring fundamental aspects of solid- state and statistical physics. Here we report the discovery of a giant nonlinear resistance effect in the conducting organic salt(2) theta- ( BEDT- TTF) 2CsCo( SCN)(4), the voltage- current characteristics of which are essentially the same as those of a conventional thyristor. This intrinsic organic thyristor works as an inverter, generating an alternating current when a static direct- current voltage is applied. Whereas conventional thyristors consist of a series of diodes ( their nonlinearity comes from interface effects at the p- n junctions), the present salt exhibits giant nonlinear resistance as a bulk phenomenon. We attribute the origin of this effect to the current-induced melting of insulating charge- order domains, an intrinsically non- equilibrium phenomenon in the sense that ordered domains are melted by a steady flow.