In Finland during 2003-2005, large numbers of nematodes, tentatively identified as Setaria sp., were observed in the peritoneal cavity of reindeer Rangifer tarandus during a peritonitis outbreak associated with poor body condition. The morphology of worms recovered from slaughtered reindeer is described based on light and scanning electron microscopy. Worms were confirmed as belonging to the superfamily Filarioidea by the presence of microfilariae-producing females and the size and the shape of the male spicules. Further classification to the genus Setaria was based on the location of worms in the peritoneal cavity and the morphology of spicules and the peribuccal crown. The present measurements correlate with those in the description of Setaria tundra by Rajewsky (1929) and only minor differences were present compared with descriptions of key parts of identification of S. yehi from the American white-tailed deer Odocoileus virginianus and mule deer O. hemionus. Current morphological findings support our earlier studies employing a polymerase chain reaction and confirm the occurrence S. tundra in reindeer in Finland.