The magnetic behavior of epitaxial La0.7Ca0.3MnO3 (LCMO) films grown on SrTiO3, with thickness down to 2.4 nm, has been analyzed and correlated to the magnetoresistance (MR). Below about 10 nm the canonical ferromagnetic order switches to a distribution of superparamagnetic clusters. These ultrathin films show a huge MR (4000%) at T-c and, at 5 K, the MR is 300 times that of the ferromagnetic films' value. Metallic conductivity is observed below T-c down to the blocking temperature (T(B)approximate to 65 K) but a strong upturn appears below T-B. These findings can be explained by the ordering of uncoupled superparamagnetic clusters and their blocking at lower temperature, together with the collapse of the magnetic domain walls which are substituted by very small Mn magnetic clusters (few Mn) probably localized at the structural domain boundaries. (C) 2005 American Institute of Physics.