An instability driven by anisotropy in velocity space has been studied in a hot electron plasma created by adiabatic compression in a pulsed magnetic mirror. Early in the compression history of the plasma, bursts of radiation occur, with frequencies below the electron cyclotron frequency and its harmonics, with no apparent loss of plasma. The anisotropy in velocity space of the plasma was determined by comparing the measured axial plasma distribution with theoretical profiles. The observed values of plasma density (cold plus hot), temperature and anisotropy are used to find the unstable modes of the complete electromagnetic dispersion relation for all angles of propagation. The fastest growing mode early in the compression cycle is electrostatic occurring below the second harmonic; here, ωp/Ω (cold) = 1.5, ωp/Ω (hot) = 0.35. Later in time, ωp/Ω (cold) = 0.5, ωp/Ω (hot) = 0.25, and the fastest growing mode is electromagnetic, occurring below the fundamental. These predictions are in agreement with the observed spectrum and polarization measurements.