Abstract
A fluid of charged particles embedded in a neutralising background is considered and, in contradistinction with ordinary fluids, its isothermal compressibility is defined from the k4-term of the small-wavevector expansion (k to 0) of the static structure factor S(k). This compressibility can also be written as a thermodynamic derivative but, contrary to what happens for ordinary fluids, it is not a positive semidefinite quantity, in agreement with the recent numerical results obtained for charged fluids.