Stropharia caerulea, commonly known as the blue roundhead, is a species of mushroom forming fungus in the family Strophariaceae.

It is a somewhat common species found in Europe and North America, where it grows as a saprophyte in meadows, roadsides, hedgerows, gardens, and woodchip mulch.

The fruit bodies (mushrooms) of Stropharia caerulea feature a greenish-blue cap sparsely covered in white flecks of veil at the margin, and a sticky, glutinous surface texture.

It is similar in appearance to a less common relative Stropharia aeruginosa, but that species has a more robust ring on its stipe, more scales on the cap, and darker gills with white edges.

In 1953, the Finnish scientist Risto Tuomikoski observed that the well-known Stropharia aeruginosa had a lookalike species that was characterized by a brown spore print, an indistinct, temporary ring on the stipe, and chrysocystidia[nb 1] lining the gill edges.

This usage is not considered valid according to nomenclatural rules, because it was used by Christian Hendrik Persoon to refer to another species in his 1801 Synopsis methodica fungorum, which is a sanctioned work.

[9] Although psilocybin had once been reported from S. caerulea,[10] this was almost certainly an error as subsequent analyses have not revealed any traces of the substance in the fruit bodies.

[12] French mycologist Régis Courtecuisse has called the mushroom "verdigris agaric",[13] but numerous other authors use this name to refer instead to Stropharia aeruginosa.

[3][nb 3] Spores typically measure 8.0–9.0 by 4.0–5.5 μm, and have an ellipsoid to oblong to ovoid shape, depending on the viewing angle.

It is distinguished from the latter by the well-developed ring zone on its stipe, darker gills with white edges, and more numerous whitish scales around the cap margin.

[21] Fruitbodies of Stropharia caerulea form mycelial cords–rootlike structures consisting of a dense mass of hyphae–which create extensive underground networks that move nutrients and allow the fungus to "forage" for resources.