Acorn barnacles were assumed to have a self-thinning exponent close to -3/2 widely acknowledged for plant populations. However, space and food competition mechanisms would lead to a different exponent. Different recruit densities between tidal levels should result in different geometry of space occupation with adult individuals morphologically different and a different biomass-density relationship. Using Tetraclita squamosa, an intertidal barnacle, we investigated the biomass-density relationship along an intertidal gradient to study self-thinning. The self-thinning slope was steeper than the -3/2 power and did not differ among lower tidal levels with an intercept significantly higher in the low intertidal than in the mid-intertidal, whereas there was no significant relationship in the high intertidal. Growth in height of crowded barnacles along with the weaker effect of competition for food may retard mortality and result in a self-thinning slope steeper than -3/2. Our results suggested that self-thinning exponent in barnacles is steeper than -3/2 found for plant populations and allometric growth may not be the only factor causing this deviation from the “self-thinning law”. As one progresses higher in the intertidal, self-thinning processes disappear owing to reduced competition.