The broad spectrum of radiation from first-order stimulated Raman scattering in glass fibres leads to a spread of transverse Stokes fields in a single-mode fibre. A general expression for the effective core area (ECA) for the laser-pump ith longitudinal Stokes-mode interaction has been found. Computations of the variation of ECA withV-value are presented, together with a single ECA for use when computing the total Stokes power.