The free jet millimeter wave spectrum of the pyridazine-argon molecular complex has been investigated in the 60-75 GI-Iz frequency range, From the information contained in the rotational spectrum the equilibrium position of the argon atom with respect to the center of mass of pyridazine (x(e) = -0.368, y(e) = 0.0 and z(e) = 3.427 Angstrom, respectively) and the two bending and stretching force constants (f(xx) = 0.211, f(yy) = 0.214, f(xx) = 2,052, N m(-1), respectively) have been obtained.