An .alpha.-indanthrone pigment having improved tinctorial and technological properties is produced by swelling .gamma.-indanthrone in sulfuric acid of from 73 to 80 percent strength by weight at from 20.degree. to 50.degree. C. in the presence of from 0.1 to 20% by weight, based on indanthrone, of a hydroxylammonium salt, a hydrazinium salt, amidosulfonic acid, urea, a salt of urea, formaldehyde or paraformaldehyde or a mixture of these. After precipitation in water which may or may not contain a surfactant, at below 50.degree. C., an .alpha.-indanthrone pigment of high tinctorial strength is obtained, which gives clear and pure colorations and is easily dispersible.