The action spectrum for photoinduction of pileus formation in Favolus arcularius (Fr.) Ames. shows four maxima, at ca. 375-385, 420, 446, and 480 nm, and minima near 365, 400, 435, and 470 nm. Wavelengths above 510 nm were ineffective. The maxima coincided with four of the six maxima for primordium formation of this fungus, but the two maxima for the latter around 398 and 514 nm could not be detected. The photosensitive region for the initiation of pileus formation was localized in the apical region of the stipes.