Canine distemper is uncommon in the pet trade in the United States, in large part due to effective vaccines against Canine distemper virus (CDV). This is a report of CDV affecting 24 young dogs of multiple breeds shortly after sale by 2 pet stores in Wyoming during August–October 2010. Cases were diagnosed over 37 days. Diagnosis was established by a combination of fluorescent antibody staining, reverse transcription polymerase chain reaction, negative stain electron microscopy, and necropsy with histopathology. Viral hemagglutinin gene sequences were analyzed from 2 affected dogs and were identical (GenBank accession no. JF283477). Sequences were distinct from those in a contemporaneous unrelated case of CDV in a Wyoming dog (JF283476) that had no contact with the pet store dogs. The breeding property from which the puppies originated was quarantined by the Kansas Animal Health Department. Puppies intended for sale were tested for CDV. Canine distemper was diagnosed on site in November 2010. At that point 1,466 dogs were euthanized to eliminate dispersal of the disease through commercial channels. The investigation underscores the risks inherent in large-scale dog breeding when vaccination and biosecurity practices are suboptimal.