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A demographic study of Cystoseira humilis Kützing present in a tidepool situated on the rocky shore south of Livorno (Ligurian Sea) was carried out. In order to assess density, growth, reproduction, mortality and biomass of the population, the plants, present in three permanent marked squares which had been randomly chosen, were tagged and monitored over three years. The results suggest that the C. humills population displayed periods characterized by high mortality and low recruitment alternating with periods characterized by low mortality and high recruitment. During the first year of the study the greatest abundance and growth of plants was recorded in spring, while a population decline was observed in the autumn-winter period. New plants appeared between October and April. Density variations were linked to seasonal fluctuations in recruitment and mortality. The seasonal trend of biomass was related to changes in density and plant height. In the second year there was a marked decline in the entire population of C. humilis due partly to absence of recruitment and partly to the high mortality rate during the winter-spring period affecting adult and juvenile plants. This high mortality coincided with an appreciable decrease both in frond length and stem number of the surviving plants, resulting in a marked decrease in biomass. In the third year the resulting availability of substratum facilitated recruitment. The development of reproductive structures showed a seasonal trend throughout the three years of the study. The greatest frequency of reproductive individuals occurred in July, while in January only sterile plants were found. [ABSTRACT FROM AUTHOR]