An outbreak of distemper was observed in beech martens (Martes foina) in the eastern part of Flanders(Belgium) for the first time. The clinical and pathological findings were consistent with other outbreaks describedin mustelids in Europe. Using reverse transcriptase polymerase chain reaction, morbillivirus RNAwas detected in an organ homogenate inoculated on Vero.dogSLAM cells. The virus was sequenced and wasidentified as a canine distemper virus (CDV) strain, hundred percent identical to an earlier isolate from amarten from Germany. After a period of nearly complete absence of beech martens in Flanders due to intensiveprosecution, their population density has increased substantially in the last decennia. Although the exactmechanisms behind the observed population changes stay unclear, spread of CDV from central Europe followingdispersion of beech martens is indicated by our findings. Further CDV spread could negatively impacton the highly threatened pine marten (Martes martes) and the decreasing polecat (Mustela putorius) populationsin Flanders.