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 Utria 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 Utria 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