Arachnomyces flavidulus is a species of ascomycete fungus discovered in 1912 by botanist Carlo Luigi Spegazzini.

Attached to the fruiting body is a subiculum (wool-like mycelium growth),[3] very thin and loose, formed by irregularly branched slender hyphae (1.5-2 μm).

The perithecia are scattered, globose or globose-depressed, 250-500 μm in diameter, lacking an opening or ostiole, of yellow color, fragile, densely covered in yellowish brittle hairs of thinly membranous indistinct composition.

The asci are subglobose, tiny, clustered, measuring 12 μm in diameter, with rapidly flowing octospores.

[2] A. flavidulus was listed as a doubtful species since 1970 because it produces ascospores that differ from other Arachnomyces members.