Protogaster rhizophilus is only known from its description, from specimens collected in 1895. It is not possible to determine current population size so the species is assessed as Data Deficient (DD).

P. rhizophilus is the only member of the monotypic genus Protogaster and the family Protogastraceae, which were erected to accommodate this species based on its distinct morphological characteristics (Zeller, 1934). No phylogenetic studies or molecular data is available.

Only known from one location in Maine in the United States of America (Zeller, 1934).

The species is only known from collections made in 1895 in one location (Zeller, 1934). It may be a rare species, but the number of records is also certainly limited by its very inconspicuous fruitbodies, which are both hypogeous and small (0.1 to 0.5 mm in diameter). Given that it was found in association with Viola sp. roots, it may also have a large area of appropriate habitat and wider distribution. Without targeted search efforts it is not possible to estimate current population size.

Hypogeous species, found growing in association with roots of cultivated Viola sp., one to three inches bellow the surface. (Zeller, 1934)

Unknown.

Further research is needed to determine habitat preference, distribution and population size for this species.