Accurate size measurements are fundamental in characterizing the population structure and secondary production of a species. The purpose of this study was to determine the best morphometric parameter to estimate the size of individuals of Capitella capitata (Fabricius, 1780). The morphometric analysis was applied to individuals collected in the intertidal zones of two beaches on the northern coast of the state of São Paulo, Brazil: São Francisco and Araçá. The following measurements were taken: the width and length (height) of the 4th, 5th and 7th setigers, and the length of the thoracic region (first nine setigers). The area and volume of these setigers were calculated and a linear regression analysis was applied to the data. The data were log-transformed to fit the allometric equation y =axb into a straight line (logy = loga + b * logx). The measurements which best correlated with the thoracic length in individuals from both beaches were the length of setiger 5 (r2 = 0.722; p<0.05 in São Francisco and r2 = 0.795; p<0.05 in Araçá) and the area of setiger 7 (r2 =0.705; p<0.05 in São Francisco and r2 = 0.634; p<0.05 in Araçá). 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