Spatio-temporal variability in the population structure of long-spined black sea urchin, Diadema antillarum, was investigated at two islands (Gran Canaria and Tenerife) in the Canarian Archipelago over a year. The mean abundance of D. antillarum across all counts was 2.70±0.07 ind m⁻² (mean±SE, N=1440), while the mean biomass was 105.76±3.75 g m⁻² (mean±SE, N=1440). Abundances and biomasses of D. antillarum differed consistently between vegetated and unvegetated bottoms; however, locations within each habitat at each island fluctuated following different trends. For the overall study, mean densities and biomasses in the barren locations varied between 3.36–6.97 ind m⁻² and 93.76–405.13 g m⁻², respectively; while mean densities and biomasses in the algal stand locations varied between 0–0.33 ind m⁻² and 0 – 7.34 g m⁻², respectively. Striking differences existed in the size-structure among locations; however, larger size-classes (test diameter >3.5 cm) were present at all locations, and usually dominated in terms of abundance. The majority of individuals in the algal stands were large-sized, probably as a result of the high abundance of the most palatable food. In contrast, small-sized individuals (test diameter <1.5 cm) only occurred in the barren habitat, suggesting that recruitment of D. antillarum could be favoured by the presence of high densities of congeners, as a way to decrease the risk of predation.