P>The applied fields in previous studies of goethite's magnetic properties were often not sufficiently high to reach reversible magnetization behaviour of the hard magnetic iron oxide (alpha-FeOOH). Existing data of its magnetic anisotropy constant are fairly scattered that hampers the further development of more elaborate models of remanence acquisition, magnetic susceptibility behaviour or trends in microcoercivity. Classic rock magnetic properties (magnetization of remanence M(r), saturation magnetization M(s) and coercive force B(c)) and the first anisotropy constant K on six goethite natural crystals have been determined at room and low temperature using hysteresis up to 9 T. After a detailed analysis of the magnetic properties of the samples, a mean value of K = 90 +/- 20 Jm-3 has been evaluated at room temperature and K = 210 +/- 40 Jm-3 at 5 K for pure goethite.