SUMMARYThe action spectrum for the phototropical curvature of the coremia ofP. isariiformewas determined by means of unilateral illumination. The I50(Λ), that is the light intensity at wavelength Λ at which 50% of the coremia react positively, was calculated from a series of experiments with the help of the probit analysis. The I50(Λ), the 95% confidence interval, and the relative quantum efficiency were calculated and a plot of the values gave the action spectrum. This spectrum is quite different from the action spectra for sporulation and initiation of coremia in the same fungus. However, with peaks at 425 nm, 450 nm and 476 nm, it resembles rather well the action spectra for other light‐sensitive fungi. Therefore it might be assumed that inP. isariiformethe same photoreceptor as in other fungi is active. The great difference between the three action spectra remains to be explained. Possibly a screening pigment causes the distortion of the action spe