  We observe the condensation of magnon excitations within an $F=1$ $^{87}$Rb
spinor Bose-Einstein condensed gas. Magnons are pumped into a longitudinally
spin-polarized gas, allowed to equilibrate to a non-degenerate distribution,
and then cooled evaporatively at near-constant net longitudinal magnetization
whereupon they condense. We find magnon condensation to be described
quantitatively as the condensation of free particles in an effective potential
that is uniform within the ferromagnetic condensate volume, evidenced by the
number and distribution of magnons at the condensation transition. Transverse
magnetization images reveal directly the spontaneous, inhomogeneous symmetry
breaking by the magnon quasi-condensate, including signatures of Mermin-Ho spin
textures that appear as phase singularities in the magnon condensate
wavefunction.
