Fast reactor parameters are compared as obtained either by a detailed fine group calculation where the scattering resonances of light and intermediate elements are well accounted for, or by coarse group calculations (∼25 groups). These fundamental mode calculations are performed with cross sections derived from the same basic data in order to check only the approximations of the calculational methods. The parameters considered are , , , one-group cross sections, danger coefficients, and the spatial heterogeneity effect. The properties which can be reliably evaluated with few groups are indicated. The fine group calculations are performed with the PROMETHEE code developed at Cadarache, using a lethargy mesh (usually about 2000 groups).