Three populations of wild foxes were sampled for serum neutralizing antibody to calcivirus (San Miguel sea lion virus) types 1-5. Neutralizing activity was detected in serum from gray foxes [Urocyon littoralis] resident on Santa Cruz Island, California, USA, but not in Arctic foxes [Alopex lagopus] from Alaska, USA. The results indicate that foxes may be naturally infected with caliciviruses, but their role in the transmission cycle is unknown.