A plasma diagnostics analysis is reported which looks ar the experimental spectrum of the sonoluminescence emitted by a single argon bubble oscillating nonlinearly in an acoustic field. Under the hypothesis that the bubble is mainly filled withe atoms able to participate in the plasma, an order-of-magnitude estimation of the electron density (N-e) associated with the intracavity medium gives N-e approximate to 10(25) m(-3), with an electronic temperature estimated to be about 20 000 K, perhaps more. This analysis suggests that the conditions at the root of single-bubble sonoluminescence are highly energy charged and may be compatible with a sparklike process. The plasma developed inside the argon bubble is assumed to be in local thermodynamic equilibrium.