Rhyssomatus subtilis Fiedler (Coleoptera: Curculionidae) is an important pest of the soybean crop in northwestern Argentina. Few studies have been made on specific parameters of its life history and ecology. The aim of this study was to determine the number of larval stages of R. subtilis. One thousand and eighteen larvae were collected from soybean plants during 2 yr (2011 and 2013), and head capsule width of each larva was measured. For analysis of data, the Hcap program and Dyar's rule were used. The Hcap program showed 4 different peaks in the frequency distribution of the head capsule widths. This result also agreed with Dyar's rule that revealed a perfect geometric larval growth pattern for each instar by regression analysis. The excellent fit to a linear model, indicates that no instar was overlooked. This research identified 4 instars for R. subtilis.