The presence of two (or more) species of ions in an inhomogeneous magnetically confined plasma can present opportunities for a new class of drift-type modes to become unstable. When the thermal velocity of a minority ion species (the impurity) is much smaller than that of the primary species, waves with parallel phase velocities intermediate between the two thermal velocities can be driven unstable by a source of the free energy (for example density or temperature gradient) and by Landau damping. Their linear stability theory and quasi-linear particle and heat transport are discussed in the context of shearless slab geometry. Also discussed is the simultaneous appearance of two unstable modes, one propagating in the ion diamagnetic direction and the other in the electron direction. When the effective impurity concentration is finite, the threshold for the ion temperature gradient instability is dramatically increased
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