Known definitive hosts for T. californiensis are the dog, deer, cat, horse, sheep, coyote, bear, and man; locality records include California and adjacent areas of Oregon and Nevada. While developmental forms of the worm were found in experimentally-infected, laboratory-reared Fannia canicularis (Diptera: Muscidae), similar developmental forms of a nematode, believed to result from natural infection, were found in specimens of F. benjamini collected near San Bernardino, California, an endemic area. This is the 1st report of intermediate hosts for T. californiensis or for any sp. of this genus in the U. S., and it agrees with Russian work on 3 spp. of Thelazia for which spp. of Muscidae were found to be intermediate hosts.