ABSTRACT. – Since the opening of the Suez Canal in 1869, a number of marine species have entered the Mediterranean Sea from the Red Sea. The seagrass Halophila stipulacea was among these introduced species. In the course of the past century, H. stipulacea developed extensive meadows in the eastern Mediterranean basin and is now extending its distribution into the western basin. As little is known of their vegetal assemblage, a temporal study of a meadow was carried out off the eastern coast of Sicily throughout a year cycle. The associated algal community was dominated by epiphytes followed by rhizophytic species. Species diversity showed significant variations over time with a maximum in October and a minimum in April. Changes in species diversity and community structure were positively correlated with the yearly cycle of seagrass’ cover. The seagrass bed was also populated by the introduced rhizophytic green alga Caulerpa racemosa, representing the second dominant vegetal species after H. stipulacea. A comparison between the associated algal assemblages of this H. stipulacea meadow and two other contiguous ones of seagrass Posidonia oceanica and Cymodocea nodosa, respectively, showed significant differences in species composition.