The propagator is evaluated for the situation of a non-interacting scalar field at non-zero temperature in curved space-time. The coincidence limit is taken and the corresponding effective Lagrangian is constructed. Fluctuations are determined using a background metric which satisfies the Einstein equations. The resulting energy-momentum tensor is found to be that for an imperfect fluid. Metric fluctuations are found to be unstable (for most values of the cosmological constant) with an exponential growth characterized by a Jeans mass squared which is twice the classical value. Comparisons with previous calculations are also made.