  We report here the first direct observations of a well ordered vortex lattice
in the bulk of a La(2-x)Sr(x)CuO(4+d) (La214) single crystal (slightly
overdoped, x=0.17). Our small angle neutron scattering investigation of the
mixed phase reveals a crossover from triangular to square coordination with
increasing magnetic field. The existence of an intrinsic square vortex lattice
has never been observed in high-temperature superconductors (HTSC), and is
indicative of the coupling of the vortex lattice to a source of anisotropy,
such as those provided by a d-wave order parameter or the presence of stripes.
