Iron, Pb, Mn, Zn, Cu, Co, Ni, Cr, Cd, Na, K, Ca and Mg concentrations were determined in leaves, stems and roots/rhizoids of Cymodocea nodosa, in seawater and in sediment from monthly collections from the Thermaikos Gulf (Greece). Statistical analysis demonstrated that only Pb concentrations in the leaves and Co concentrations in the stems displayed significant seasonal variation. However, the variations of Cu, K and Na in the roots/rhizoids between the three seasons (spring, summer, autumn) were also significant. The concentrations of Pb in the leaves and Cu in the roots/rhizoids could, generally, be included in one group of seasonal variation with high values in winter and autumn and low in summer or spring, whereas Co in the stems and Na and K concentrations in roots/rhizoids followed the opposite pattern. The variation of the first group was discussed in relation to the growth dynamics of the plant and of the second seasonal variation with tissue age and with the role of metals in physiological processes. The iron and Mn content in the leaves and Cu in the roots/rhizoids of C. nodosa were negatively correlated with their total concentrations in the sediment and Ca dissolved in the seawater was correlated with the concentrations in the stems. The concentrations of Pb, Cu, Cd and Na in the leaves were correlated with those in the stems; Pb, Cu and Ni concentrations in the stems were correlated with those in roots/rhizoids and Pb in the leaves with those in roots/rhizoids. Leaves of C. nodosa had higher Zn, Cu, Cd, Mn, Ni, Co, K and Ca concentrations than in the stems, whereas roots/rhizoids had higher values of Cu, Cd, Fe, Ni, Co, Cr and K in comparison with stems. The Co and Zn content in the leaves was higher than in roots/rhizoids. Cymodocea nodosa in Thermaikos Gulf could be regarded as a bioindicator for Mn and Cu. Leaves seem to constitute a better bioindicator for Mn, while the best collection time for the plant to be used as a bioindicator for those metals is autumn.