In the context of the abnormal mass mortality of mussels in France since 2014, Flow CytoMetry (FCM) was used in 2015 and 2016 to study the DNA content and cell cycle characteristics of hemic circulating cells collected from 2000 mussels. The mussels were sampled from 12 wild and cultivated blue mussels stocks distributed along the French Atlantic coast from the south Brittany to Pertuis Charentais areas. During these surveys, various genetic abnormalities were frequently detected, and ploidy characteristics revealed contrasting profiles that corresponded to respective contrasting sanitary status, i.e. healthy mussels with high cytogenetic quality (HCQ) versus diseased mussels with low cytogenetic quality (LCQ). In the present work, FCM and hemocytology cell monolayer techniques were combined in order to determine the putative causes of the observed genetic abnormalities that were significantly associated with mortality levels. FCM and cell monolayer approaches permitted the definition of new threshold values delimiting HCQ mussels from LCQ ones. FCM histograms of mussels from the HCQ group showed one single or a largely dominant population of diploid (2n) nuclei and a large majority of normal hemocytes. Hemolymph cell-monolayer analyses showed predominantly acidophil granulocytes characterized by nuclei of normal size and a large cytoplasm with numerous granulations. In contrast, FCM histograms for the LCQ group showed, in addition to the normal diploid (2n) nuclei, populations of nuclei that displayed aneuploidy patterns in a broad ploidy range, including diploid-triploid (2–3n), tetraploid-pentaploid (4–5n) and heptaploid-octaploid levels (7–8n).  The corresponding hemolymph cell-monolayer showed cellular features characteristic of disseminated neoplasia disease with frequent abnormal anaplastic cells that exhibited noticeable numbers of mitotic figures with both normal and aberrant chromosomes segregation patterns. These neoplastic cells were a rounded shape with a reduced, granulation-free cytoplasm and large (11–12 μm) to very large (up to 21 μm) round or ovoid nuclei that correspond to the 4–5n and 7–8n nuclei previously detected by FCM analyses. These characteristics suggest that the genetic abnormalities detected by means of FCM were related to an ongoing neoplastic process that is affecting blue mussels in France, at least since the onset in 2014 of the mortality that heavily impacted French blue mussels stocks