A six-month-old beagle was presented with a three-month history of failure to gain weight, lethargy, intermittent vomiting and seizures. Hypoglycaemia, portosystemic shunt, lead intoxication, gastrointestinal diseases and hereditary metabolic disorders were considered. Laboratory test results of low serum cobalamin (Cbl) concentrations, anaemia, leucopenia and methylmalonic aciduria while the dog was receiving a balanced commercial canine diet were suggestive of a congenital selective Cbl malabsorption. Treatment with repeated injections of parenteral cyanocobalamin (CN-Cbl) at 50 μg/kg every two weeks corrected the Cbl-deficient state and reversed all the clinical abnormalities. Selective Cbl malabsorption has previously been described in giant schnauzers and border collies and represents a unique readily treatable hereditary metabolic disorder.