J.-Y. Chemin proved the convergence (as the Rossby number ε goes to zero) of the solutions of the Primitive Equations to the solution of the 3D quasi-geostrophic system when the Froude number F = 1 that is when no dispersive property is available. The result was proved in the particular case where the kinematic viscosity ν and the thermal diffusivity ν ′ are close. In this article we generalize this result for any choice of the viscosities, the key idea is to rely on a special feature of the quasi-geostrophic structure. 1 2 (global for small initial data). We refer to Remark 6 for the notion of well/ill-prepared initial data.Remark 2 As explained in [5,11] two distinct regimes have to be considered regarding the eigenvalues of the linearized system: the case F ∈]0, 1[ where the system features dispersive properties, and the case F = 1, with simpler operators but where no dispersion occurs. In the dispersive case (see [6] for weak solutions, [5] for strong solutions), using the approach developped by Chemin, Desjardins, Gallagher and Grenier in [14,15,16] for the rotating fluids system, we manage to filter the fast oscillations (going to zero in some norms thanks to Strichartz estimates providing positive powers of the small parameter ε) and prove the convergence to the solution of System (QG) below (even for blowing-up ill-prepared initial data as in [7,11], less regular initial data as in [8] or with evanescent viscosities as in [9]). On the contrary when F = 1 no dispersion is available and only well-prepared initial data are considered. In addition, in [13] the asymptotics are obtained only when ν and ν ′ are very close, in [20] is dealt the inviscid case. We refer also refer to [21,22,24] for results in other context such as periodic domains for example where there is no dispersion, and resonences have to be studied.