We encountered two cases of human intraocular infection with mesocercariae of Alaria (Trematoda), involving unrelated Asian men who had unilateral decreased vision. Both patients had pigmentary tracks in the retina, areas of active or healed retinitis, or both, and other signs of diffuse unilateral subacute neuroretinitis. Similar, nonnematode worms were seen in the patients' retinas and vitreous, respectively, several years after apparent infection. The worm in Case 1 was analyzed from projected fundus photographs and diagnosed as an Alaria mesocercaria on the basis of its shape, size (500 x 150 microns), and movement; it was successfully killed with laser. The worm in Case 2 was removed surgically from the vitreous and identified as A. mesocercaria, 555 x 190 microns, most likely A. americana. The probable source of infection in the patients was ingestion, in local restaurants, of undercooked frogs' legs containing A. mesocercaria. In addition to causing prolonged intraocular infection, A. mesocercaria was found to be a cause of diffuse unilateral subacute neuroretinitis, a condition previously attributed only to intraocular nematode larvae.