[en] Energy spectrum analysis of a balloon observation of Cygnus X-1 is reported. During a period of flaring activity of the source, a much softer spectrum than usual, fitted by a power law with number index 2.7, has been found. The ''normal status'' of flux is attained in a rather smooth way. During this last period the energy spectrum is well represented by a power law with number index 1.84, in agreement with other observations