The present paper reports the results of two different studies carried out to investigate interactions between naturally mixed Mediterranean populations of Caulerpa taxifolia and C. racemosa and the effects of their co-presence on native benthic assemblages. Results of the first study showed that co-occurring populations of the two algae exhibited different growth patterns, while growth was similar in single-species patches. Caulerpa racemosa did not seem to be affected by the presence of C. taxifolia, whereas the latter species stopped growing and decreased in cover in mixed beds. In the second study, macroalgal assemblage showed significant differences in total percent cover and species number between invaded and reference areas. Moreover, total percent cover, diversity and percent cover of filamentous, articulated and crustose algae were lower in C. racemosa patches than in C. taxifolia patches. In the mixed beds an intermediate situation occurred, with patterns more similar to C. racemosa patches. C. racemosa seemed to affect macroalgal assemblages more severely than C. taxifolia and, in mixed beds, effects of invasion appeared more affected by the presence of C. racemosa, though this species colonised more recently the study area.