Populations of Cystoseira spinosa, an endemic Mediterranean brown alga, are restricted to deep water at undisturbed sites. The species composition of an old stand in the Scandola Marine Reserve (Corsica) at a depth of 26–29 m reflects the dominance of Cystoseira spinosa over other brown and red algae, with an average biomass of 450 g dw m−2, and a mean density of 28.8 plants m−2. The length of the main axis was chosen as a convenient parameter related to size. The number of secondary axes increases exponentially with the length of the main axis, suggesting the existence of environmental constraints (possibly hydrodynamism), which may impose severe restrictions on the unlimited growth of the main axis. The size pattern of the Cystoseira spinosa plants from the population studied showed a log-normal distribution, with the highest frequency within size classes of 9–15 cm (main axis length), and a maximum size of 44 cm. Areas devoid of adult Cystoseira plants and lacking other erect algae had a significantly higher (8%) number of recruits than undisturbed areas. We suggest that the log-normal size distribution found in the population studied is due to an inhibition of recruitment by erect algae, and that disturbances that remove these algae are necessary to increase Cystoseira recruitment.