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Instability thresholds for parallel‐pumped magnons in a two‐sublattice ferrimagnet are derived by means of a perturbation method. Cases are considered in which the pump frequency is twice a particular magnon frequency or else the sum of two distinct magnon frequencies—one belonging to each branch of the spinwave dispersion relation. The calculations are made within the molecular field approximation for a single crystal of ellipsoidal shape, and include the effects of the volume dipolar energy as well as those due to unequal g factors of the sublattices. The thresholds appropriate to an ``unflopped'' antiferromagnet are also given when the dc magnetic field is applied parallel to the anisotropy axis. The prospects for experimentally reaching the thresholds, particularly those occurring at submillimeter wavelengths, are evaluated insofar as is possible; in such cases the possibility exists of utilizing a laser for the pump.