Bivalve transmissible neoplasia (BTN) is a leukemia-like cancer "metastasizing" by transmission of living cancer cells between molluscs. Blue mussels harbor two evolutionary lineages of BTN,MtrBTN1 andMtrBTN2, both derived fromMytilus trossulus. WhileMtrBTN1 has been found only inM. trossulusin North Pacific,MtrBTN2 parasitizes different Mytilus species worldwide, particularly in Western Europe. No targeted studies of BTN in Northern European mussels (M. edulis,M. trossulus) have been made. We searched for BTN in mussels from the Kola Bay (Barents Sea) with the help of flow cytometry of the hemolymph, qPCR with primers specific to cancer-associated alleles and sequencing of mitochondrial and nuclear loci. The species of the mussel hosts was ascertained genetically. BothMtrBTN1 andMtrBTN2 were present in our material, though their prevalence was low (~0.4%). The only instance ofMtrBTN2 was found inM. trossulus.MtrBTN1 occurred inM. trossulusand in a hybrid betweenM. trossulusandM. edulis. This finding indicates thatMtrBTN1 may potentially infect the latter species. The mtDNA haplotypes found in both lineages were nearly identical to those known from the North Pacific, but not from elsewhere. Our results suggest that they arrived in the Kola Bay fairly recently, probably with the maritime transport along the Northern Sea Route, and that the invasion was independent of that in Western Europe. A relatively young evolutionary age ofMtrBTN1 seems to suggest that it is an emerging disease in the process of niche expansion.