Although previous studies have noted the presence of the bivalve Pinna nobilis Linnaeus, 1758 in Mljet National Park, there is a lack of basic knowledge of this endangered species. The present study was initiated as part of a long-term investigation of P. nobilis density and growth with the aim of gaining a better understanding of this species´ population dynamics, as well as evaluating the efficiency of protection measures of P. nobilis in Mljet National Park. Density, population structure, growth of tagged individuals and epibionts on P. nobilis were studied using SCUBA diving in 1998 and 2000 along four transects in Mljet inlets, Adriatic Sea. The density of P. nobilis ranged from 0.20 ind/m2 on transect MLJ-9 in Malo Jezero in 1998 to 0.02 ind/m2 on transect MLJ-5 in Veliko Jezero in 2000. Individuals measured in 1998 were significantly smaller than those measured in 2000. Growth curves were constructed from data collected by tagging and re-measuring 47 individuals. A decrease in density along some of the transects and the disappearance of 24 tagged individuals in 2000 is suggestive of poaching. The results of the present study indicate recovery of the P. nobilis population in Mljet lakes, but also provide evidence for the illegal removal of this protected species.