A method is presented for predicting the genetic superiority of rainbow trout (Salmo gairdneri) selected by multistage selection for body mass at ten months of age. Trout selections are currently based on successive gradings for body width at 2, 4 and 6 months of age. Followed by successive weighings for body mass at 8 and l0 months. Such multi-stage selection distorts the initial multivariate distribution .making it difficult to describe the consequences of selection in the later stages. The selection practiced is. however, intermediate to two extreme forms of selection. Basing approximate calculations on values of input parameters intermediate to those of the extreme methods of selection is proposed.