We report the first observation of an Fe3+ maser oscillation at zero magnetic field inside a whispering gallery (WG) sapphire resonator. The described maser is new in that it operates at zero-field and with low ion concentration. At zero-field, the Fe3+ ion shows a 3-level structure related to the electron spin resonance (ESR). By applying a 31 GHz pump (vertical bar 1/2 > -> vertical bar 5/2 >), the ion operates as a maser at 12 GHz (vertical bar 5/2 > -> vertical bar 3/2 >). The maser effect is made possible by the high Q-factor (several 10(8)) of the cryogenic whispering gallery resonator. Additionnaly, the sharp cavity resonance provides short term stability. Preliminary measurements indicate a frequency stability of parts in 10(-14) (Allan deviation at 100 s), still limited by the instrument. The ultimate maser stability is still unknown.