ABSTRACT. – The abundant-centre model is a theory commonly invoked to explain species distributions. From this model, the density of a species is predicted to peak at its distribution centre (abundant-centre hypothesis), as well as maximum body size, both variables being predicted to decrease similarly towards range margins. These predictions have rarely been tested along the full distribution range of species using the same sampling effort along the range. We tested the hypotheses that density and maximum body size would peak at a species’ distribution centre and decrease similarly towards range edges by surveying intertidal barnacles throughout their full vertical range of distribution. We studied Semibalanus balanoides, which occurs between elevations closely above the lowest tides and closely below the highest tides in rocky intertidal habitats from Atlantic Canada. Analyses of data from 7 contiguous elevation zones indicated that both density and maximum body size followed a unimodal vertical trend. However, both variables peaked at a higher elevation than the distribution centre, decreasing to very low values towards the lower range margin. Several lines of evidence suggest that interspecific interactions may be limiting barnacle development at low elevations and that abiotic stress may limit development at the highest elevation zone. Overall, our study reinforces the notion that density and body size predictions deduced from the abundant-centre model represent an oversimplification of reality.