The viscoelastic properties of two series of poly(methyl methacrylates) (PMMA) with different tacticities and very narrow molecular weight distributions were measured in the molten state at different temperatures by means of oscillatory rheometry. Zero-shear viscosity eta(0), steady-state shear compliance J(e)(o), and plateau modulus G(N)(0) were obtained from the resulting master curves for the storage modulus G’ and loss modulus G ". Relaxation time spectra h(tau) were calculated by a nonlinear regularization method and could be used to determine the terminal relaxation time tau(0). The molecular weight and polydispersity dependence of these parameters agrees with existing empirical rules for linear amorphous polymers. Stereoregularity, however, has a great influence on eta(0) and tau(0). The entanglement molecular weight M(e) and the critical molecular weight M(c) seem to vary with the tacticity as well. These results are possible explanations for some contradicting or inconsistent literature data concerning PMMA.