Based on Coulter Counter data, a new method is proposed to study the influence of the polymorphous variability in size as well as in shape of the diatom species Skeletonema costatum on the physiological performances of the Manila clam Ruditapes philippinarum . Three conditions are required in order to convert volumetric data determined with the Coulter Counter into metric data, i.e. (1) stability of the pervalvar diameter of the diatom chains, (2) statistical independence between length and pervalvar diameter of the diatom chains, and (3) negligible level of coincident passages through the aperture orifice. Under these three conditions, the mean length (l) of a S. costatum chain can be related to the mean pervalvar diameter (d) of diatom chains and to the spherical diameter (D) of each channel of the Coulter Counter. The mean length (l) of a S. costatum chain is then estimated by l = 4 D super(3)/6 d super(2). Application of this analytic method is discussed.