A dyeing-stable modification (.beta.-modification) of the dyestuff of the formula (I) ##STR1## which has the X-ray diffraction diagram shown in FIG. 1 and having the characteristic reflections at the following glancing angles and relative intensities:______________________________________ [.theta.] 2.74 4.27 4.70 5.50 8.74 11.24 12.44 relative 100 72 40 23 27 32 26 intensity ______________________________________ a process for its preparation by heating the dyestuff of said formula (I) of the dyeing-unstable .alpha.-modification, characterized by an X-ray diffraction diagram having the characteristic reflections at the following glancing angles and relative intensities______________________________________ [.theta.] 3.55 4.15 5.35 9.45 10.41 12.71 13.89 relative 42 100 35 24 34 94 48 intensity ______________________________________ at temperatures of 60.degree. to 120.degree. C. in an organic solvent, and its use for dyeing fiber materials made of cellulose esters or synthetic linear polyesters.