This paper describes the distribution and patterns of spread of the red alga Lophocladia lallemandii in the Tuscan Archipelago (NW Mediterranean Sea). Lophocladia lallemandii is recorded from three islands of the archipelago. Invasive traits were only detected at Pianosa Island where the affected stretch of coast has increased from 1.2 km in 2008 to 10.7 km in 2010. The abundance of the alga was found to increase with depth and strong differences among habitats were found in shallow stands. The presence of populations of Cystoseira spp. seemed to facilitate the spread of the alga. In Cystoseira assemblages, the biomass of L. lallemandii could represent up to 44% of the biomass of Cystoseira spp. and epiphytes. The ability of L. lallemandii to colonize canopy species exposes habitats ecologically important to be invaded, and contradicts the classic concept of ecosystem resistance to biological invasions.