Notoacmea biradiata (Reeve, 1855) is an abundant intertidal limpet on the rocky shore of the Colombian Pacific coast. This paper reports a field factorial experiment on shell-size variation of N. biradiata as a response to two independent factors: intertidal gastropod density and shore height above sea level. Four rocky shores were selected on the Colombian Pacific coast, two in Gorgona Island and two in Utr?Cove. The intertidal gastropods density was determined in each rocky shore using the transect-quadrant technique. Significant differences were observed in the gastropod densities between study sites: low density in the rocky shores of Utr?Cove and high density in the rocky shores of Gorgona Island (P < 0.009). A factorial sampling was conducted to determine the effect of the density and height on the shore over shell-length, shell-height and shell-thickness. A twoway ANOVA and a least-squared regression was used to determine whether the shell-sizes differ between varying densities and shore heights. Intertidal gastropod density was found to be the dominant factor in determining basal shell length (ANOVA, P < 0.001) and shell height (ANOVA P < 0.05) changes. Thus, at low population densities the shells are larger and taller. The height on the shore did not influence the shell-size of N. biradiata (ANOVA, P > 0.30).