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Differentially rotating disks are subject to the axisymmetric instability for perfectly conducting plasma in the presence of poloidal magnetic fields (Balbus & Hawley 1991). For nonaxisymmetric perturbations, we find localized unstable eigenmodes whose eigenfunction is confined between two Alfven singularities at omega<SUB>D</SUB> = +/- omega<SUB>A</SUB>, where omega<SUB>D</SUB> is the Doppler-shifted wave frequency and omega<SUB>A</SUB> = k<SUB>parallel</SUB> nu<SUB>A</SUB> is the Alfven frequency. The radial width of the unstable eigenfunction is Delta x is approximately omega<SUB>A</SUB>/(AK<SUB>y</SUB>), where A is Oort's constant and k<SUB>y</SUB> is the azimuthal wavenumber. The growth rate of the fundamental mode is larger for smaller values of k<SUB>y</SUB>/k<SUB>z</SUB>. The maximum growth rate when k<SUB>y</SUB>/k<SUB>z</SUB> is approximately 0.1 is approximately 0.2 Omega for the Keplerian disk with local angular velocity Omega. It is found that the purely growing mode disappears when k<SUB>y</SUB>/k<SUB>z</SUB> is greater than 0.12. In a perfectly conducting disk, the instability grows even when the seed magnetic field is infitesimal. Inclusion of the resistivity, however, leads to the appearance of an instability threshold. When the resistivity eta depends on the instability-induced turbulent magnetic fields delta B as eta (mean value of delta B squared), the marginal stability condition self consistently determines the alpha-parameter of the angular momentum transport due to magnetic stress. For fully ionized disks, the magnetic viscosity parameter alpha<SUB>B</SUB> is between 0.001 and 1. Our three-dimensional MHD simulation confirms these unstable eigenmodes. It also shows that the alpha-parameter observed in simulation is between 0.01 and 1, in agreement with theory. The observationally required smaller alpha in the quiescent phase of accretion disks in dwarf novae may be explained by the decreased inoization due to the temperature drop.