Seasonal and vertical morphological variation occur in the Hawaiian populations of Pterocladia caerulescens. In the subtidal plants the main axes elongate from July to December; this correlates significantly with increased water movement and decreased light intensities. Tetraspore formation, shedding and apical decay occur following additive growth, thus shortening the axes and reducing the number of branches. This annual process, combined with the apparently continuous growth in breadth of main axes inplants older than 1 year, results in morphologically different sterile and tetrasporangial thalli. The main axes of the intertidal populations are shorter, narrower and less branched than the subtidal plants. The intertidal thalli cannot be found after January, correlating with increased average daily emersion time. They start appearing again in April–May and from July to December they have a pattern of growth and reproduction similar to the subtidal populations. No evidence of morphological differences between sterile and tetrasporangial plants can be found in the annual intertidal population.