A set of fluid equations is used to determine the stability boundaries for flute instability of a plasma in a nonuniform magnetic field, in the presence of simultaneous density(&dgr;′)and temperature(&lgr;′)gradients. The resulting surfaces in&dgr;′, &lgr;′, P,Pspace (Pproportional to plasma density) show the various conditions leading to instability for either magnetic wells or ordinary mirrors. It is demonstrated that a modified form of the fluid equations given by Varma is most appropriate for high‐energy injection plasma. A temperature gradient in the same direction and of the same magnitude as the magnetic field gradient is always destabilizing.