Endolith microboring infestation in bearded horse mussel Modiolus barbatus collected from Mali Ston Bay, Croatia during 2010 was studied for incidence and distribution of damage in bivalve shells and in relation to shell length and condition index. Macroscopic changes in colour and thickness of the inner shell surface were investigated in order to determine endolith presence and host response. Results showed a significant percentage of over 93% of shells to be infested by endoliths. Analyses of shells grouped by the degree of infestation showed that the majority of specimens were moderately infested, most commonly within the central area of the shell. Microboring endoliths were dominated by cyanobacteria Plectonema (Leptolyngbya) terebrans, Mastigocoleus testarum, Hyella caespitosa and by the green algae Phaeophila sp. and Ostreobium quekettii. In addition, characteristic scars were carved into the host shell by the boring bivalve Rocellaria dubia in over 40% of sampled shells. SEM photographs revealed added shell layer deposition in infested specimens; in some cases, a prismatic layer is formed on the inner shell surface. This study shows that the incidence of infestation increases with shell length and that the degree of endolith infestation has adverse effects on the condition index.