  We study the quark number susceptibility of the hot quark-gluon plasma at
zero and non-zero quark number density, using lattice Monte Carlo simulations
of an effective theory of QCD, electrostatic QCD (EQCD). Analytic continuation
is used to obtain results at non-zero quark chemical potential. We measure both
flavor singlet (diagonal) and non-singlet (off-diagonal) quark number
susceptibilities. The diagonal susceptibility approaches the perturbative
result above 20T_c, but below that temperature we observe significant
deviations. The results agree well with 4d lattice data down to temperatures
2T_c. The off-diagonal susceptibility is more prone to statistical and
systematic errors, but the results are consistent with perturbation theory
already at 10T_c.
