A method is developed for obtaining multigroup cross sections from the BN approximation in a way similar to that of the PN theory. It is shown that from a programming point of view both formalisms are very close. Moreover, it is sometimes advantageous to considerably reduce the resulting matrix for the BN approximation to the size of the matrix for the B0 approximation by eliminating the higher angular moments. Although the new method requires less memory and computing time, the resulting system of equations is mathematically equivalent.