Stability of multi-field cosmological solutions in the presence of a fluid

We explore the stability properties of multi-field solutions in the presence of a perfect fluid, as appropriate to assisted quintessence scenarios. We show that the stability condition for multiple fields ${\varphi }_i$ in identical potentials $V_i$ is simply $d^2{V}_i\mathrm{/}d{\varphi }_i^2>0$, exactly as in the absence of a fluid. A possible new instability associated with the fluid is shown not to arise in situations of cosmological interest.