Epizootiologic outbreaks of disseminated neoplasia have been reported in association with massive mortalities of various bivalve species. In cockles, Cerastoderma edule, this pathological condition was described in Ireland and France. Since 1997, different populations affected by this pathology have been detected in Galicia (NW Spain). Transmission electron microscopy allowed the visualization of virus-like particles in neoplastic cells, resembling a retrovirus-like agent. To confirm this hypothesis, we used a commercial kit for detection and quantification of reverse transcriptase (RT) activity, based on the use of bromo-deoxyuridine triphosphate (BrdUTP) and a BrdU binding antibody conjugated to alkaline phosphatase. In addition, we developed a product-enhanced RT assay using RNA of hepatitis A virus as a template. These two assays showed positive RT activity in 90.9 and 81.8% of samples, respectively, from cockles displaying disseminated neoplasia as determined by light microscopy. These results strongly support the hypothesis of retroviral etiology for this pathological condition.