This paper provides an outline of a mathematical method to analyse overlapping normal distributions. Separations of mixtures of normal distributions is frequently required in fisheries biological analysis, as in the case of length frequency distribution of fishes. Several methods are available for this. The method presented in this paper was originally developed by V Hasselblad and has been cast in its present form by D Pauly and N David. It was originally published as ICLARM contribution No. 193.