Abstract A 9year-old male, neutered cat with a history of a sudden onset of lethargy, anorexia and respiratory distress was presented in a veterinary practice in Lucca, Italy. A clinical examination revealed that the cat was severely dehydrated, and had pale mucous membranes and tachypnoea. No pain or discomfort was detected at the time of physical examination. The cat was administered fluids, antibiotics and supportive therapy, but died overnight. The owner of the cat requested for a post mortem examination to be conducted. At necropsy, acephalic structures, consistent with proliferative tapeworm (cestode) larvae, were detected in the thoracic cavity on pleural surfaces. As these larvae could not be identified to genus or species by microscopy, a PCR-based sequencing-phylogenetic approach was used. Part of the cytochrome c oxidase subunit 1 gene was PCR-amplified from genomic DNAs from five individual larvae and sequenced; all five sequences obtained were identical. This consensus sequence was aligned (over 355 nucleotide positions) with homologous sequences representing a range of cestodes (including Echinococcus granulosus, Echinococcus multilocularis, Hymenolepis microstoma, Mesocestoides spp. and Taenia saginata) from previously published studies and then subjected to phylogenetic analysis. The sequence representing the larval cestode from the affected cat grouped, with strong statistical support, with those representing Mesocestoides corti and Mesocestoides lineatus. Therefore, a definitive diagnosis of pleural proliferative larval mesocestoidiasis could be made. This study illustrates the value of using molecular tools to directly assist clinical and pathological investigations of cestodiases of animals.