The correlations among the total length, pré-umbonal length, width and height of the shell of Neocorbicula limosa (Maton, 1811), based on the examination of 100 shells from the Rio Guaíba basin are established. The radio index among these measures was calculated and the maximum, intermediary and minimum values are presented. As a result of data analysis it was pointed out that the relationship among the measures are positive and there is no allometry. It is also suggested that the proportion indexes could be used as helpful diagnostic character in conchologic studies.