A spin-flop transition is observed when a field of 20 T is applied parallel to the h axis of a specimen of natural goethite at 4.2 K. The gwthite orders in a four-sublattice antiferromagnetic structure with the sublattice magnetizations inclined at i I 3 ' to b, and an iron moment of 3.90 pg. There is a small net ferromagnetic moment of 0.004 pg along b which has the eRect of more than doubling fhe threshold field for spin flop. The anisotropy field is deduced to be 0.1 1 T. cotresponding to a uniaxial anisotropy energy K = 6 x IO4 J d. The origin of the weak moment is discussed. and it is suggested that mode superparmgnefism may arise in non-collinear antiferromagnets when two transverse spin configurations xe effectively degenerate.