A new species of Blyttiomyces, B. rhizophlyctidis, is described and discussed. It is readily distinguished from all other previously known species of the genus by its angular or tubercular sporangia, its yellowish orange sporangial content, its multiguttulate zoospores, it sproduction of abundant resting spores with a pale yellowish orange globule, and its hyperparasitic habit on another chytrid, rhizophlyctis rosea. The reaction of the host chytrid to the parasite is one of partial accommodation or total susceptibility and ultimate death, depending on the stage of development at which it is parasitized. A further broadening of the present generic concept of Blyttiomyces is suggested. Attention is called to a very real possibility of misconstruing a Blyttiomyces lacking resting spores for a Phlyctochytrium, and vice versa. Phlyctochytrium vaucheriae Rieth is transferred to Blyttiomyces under the binomial B. vaucheriae (Rieth) Dogma, comb. nov.