ALTHOUGH powdery scab disease of potatoes has been known and studied for nearly one hundred years, zoosporangia have never been described in the life-history of the causal organism, Spongospora subterranea. Zoosporangia have, however, been observed in other genera of the Plasmodiophorales by Cook1 and by me2. It therefore seemed probable that they might be found in Spongospora, especially if microscopic examinations of the roots of the host were made early in the course of the disease.