Morphological characters and growth rates of Ascophyllum nodosum were compared on fronds collected from different shore levels at one site in Strangford Lough, Northern Ireland. Both the bladders and axes of thallus tips were smaller and less variable on the upper shore than on the middle and lower shores, and the proportion of biomass invested in the production of bladders was smaller. The axes of upper shore plants were wider and flatter than on the lower shore, and the bladders were also relatively flat on the upper shore but more rounded on the lower shore. The maximum age of unbroken fronds, as estimated from the number of air bladders, was 6 years on the upper shore and 17 years on the lower shore, indicating the higher probability of breakage of plants caused by stress on the upper shore. In situ growth rates of individually marked plants along a vertical shore transect were monitored at 2- to 4-week intervals for 31 months. Length increase was highest (16 cm yr−1) in the middle of the Ascophyllum nodosum zone, and lowest (10 cm yr−1) on the upper shore. Growth was strongly seasonal with low (but not zero) growth rates during November and December, and highest growth rates in late spring and early summer. A decline in growth in mid-summer was observed at all shore levels. Growth was positively correlated with sunshine hours and total solar irradiance in winter and spring but there was no correlation with these factors during summer months. Maximum daily temperatures had a positive influence on growth in late spring, but no effect in the rest of the year. In transplantation experiments conducted in summer and winter, 80% of plants moved from the lower to the upper shore died within 3 months, whereas all transplants from the upper to the lower shore and all controls survived. Growth rates of surviving transplants at both shore levels were similar to those of control plants at the same shore level in the summer experiment. The pigment content and photosynthetic capacity of surviving plants acclimated to ambient conditions and, after 3 months, were not significantly different from those of control plants at the same shore level. It is suggested that the phenotypic acclimation of physiological characteristics and the genetic adaptation of morphological features contribute to the ability of this species to dominate such a wide band in the intertidal zone of sheltered rocky shores in the North Atlantic.