The temporal dynamics of biomass, phenology and sexual reproduction of Zostera marina were investigated during 1997 in a shallow meadow located off the eastern sea entrance of the Lagoon of Grado (northern Adriatic Sea). Total biomass and the relative contribution of the above- and below-ground compartments were measured, as well as the shoot density and phenological parameters of vegetative and fertile shoots. Flowering and seed production were also investigated and the stages of spathe maturation were recorded. Total biomass was highest in summer and lowest in autumn; the relative contribution of the above-ground compartment, mainly composed of leaf blades, was highest in summer and lowest in autumn, when it accounted for about 40% and 7%, respectively. Below-ground organs (rhizomes and roots) accounted for a higher fraction during the autumn-winter period. Shoot density and all the phenological features of vegetative shoots, except the number of leaves per shoot, changed widely throughout the year with maxima generally occurring in spring-summer and minima in autumn-winter. With regard to the sexual reproduction, the flowering of Z. marina was at the initial stage and the density of fertile shoots m −2 was lower in winter than in spring (6 vs 265 fertile shoots m −2 ), when ripe fruits occurred on spathes. In summer, the fertile shoots still present (5 fertile shoots m −2 ) were dark-brown coloured with evident signs of decay. A pollination success of 55% was observed and a potential of seed production of more than 20000 seeds m −2 was estimated in spring. The seasonal plant dynamics and the timing of the sexual reproduction of Z. marina at Grado seem to be similar to those observed in the Lagoon of Venice and in some areas outside the Mediterranean Sea. The results of this paper and the published data emphasise the great variability in the temporal dynamics, phenological features of both vegetative and reproductive shoots, and in the potential seed production of Z. marina . In this respect, the well known phenotypic plasticity of this species may account for its ability to live in variable environmental conditions. The high potential of sexual reproduction, however, is apparently in contrast with the absence of records of seedlings in the northern Adriatic Sea.