For a given space F, let
F → E∞(F) → B∞(F⟩ be the classifying fibration for fibre homotopy equivalence
classes of fibrations with fibre F. The usual theoretical construction of this fibration
offers little insight into its structure homotopically. Below we study this structure
under the hypothesis that F has homotopy concentrated in a stable range. As an
application of this study, for F a stable two stage Postnikov system determined by a
Steenrod operation, we obtain explicit descriptions of the spaces E∞(F) and
B∞(F).