Morbilliviruses pose a significant threat to marine mammal populations around the world and have been associated with multiple epizootics in pinnipeds and cetaceans. As part of a preventive veterinary medical program, five harbor seals (Phoca vitulina) in a managed collection were vaccinated with a recombinant canarypox-vectored canine distemper virus vaccine. The vaccine was evaluated for safety (by monitoring seals for local and systemic adverse effects and by testing for shedding of the canarypox vector) and efficacy (by testing for serum neutralizing antibodies). None of the seals showed signs of local or systemic adverse reactions to the vaccination. Three seals vaccinated once did not seroconvert, but the recombinant vaccine induced a persistent serum virus neutralizing titer (12 mo) in the two seals that were vaccinated twice, 1 mo apart.