Orthogonal Frobenius reciprocity, J. Algebra (to appear) Gabriele Nebe, Lehrstuhl B f\"ur Mathematik, RWTH Aachen, Templergraben 64, 52062 Aachen, Germany URL: http://samuel.math.rwth-aachen.de/~LBFM/gabi/ Abstract: A quantitative version of Frobenius reciprocity for isometries is proved. It is applied to deduce recursion formulas for the rational class of some irreducible orthogonal modules of the symmetric groups.