An X-ray spectrum of the Perseus Cluster in the energy range 1·3–16 keV has been obtained with the MSSL collimated proportional counter on Ariel 5. An emission feature has been detected at about 7 keV of strength 0.0035 ± 0.0004 photon cm–2 s–1 (equivalent width 360 ± 50 eV). The existence of this feature, which is due to Fe XXV and Fe XXVI transitions, provides strong evidence for the presence of hot plasma in the cluster. In addition the overall spectrum is well described by the bremsstrahlung emitted from an adiabatic hydrostatic atmosphere of hot gas in the gravitational potential well of the cluster.