  We have investigated the modulational instability (MI) in a negative index
media (NIM) using a new generalized model describing the pulse propagation in a
negative index material embedded into a Kerr medium. We have found that one
could control the gain of MI in a NIM by tuning the initial electric or
magnetic field amplitudes . Our model successfully recovers previously proposed
models to describe pulse propagation in NIMs exhibiting Kerr nonlinearity.
Moreover it contains a few additional terms connecting both the electric and
magnetic field envelopes in a NIM.
