Ultrasonography of a cat with diarrhoea and vomiting revealed a multi-layered, discrete linear structure within the large intestine with retention of the intestinal layers which could potentially be confused with an intestinal intussusception. The structure was ultimately expelled from the large intestine during defecation, and confirmed as a fibrinonecrotic cast. The origin of the fibrinonecrotic cast was assumed to be an intestinal pseudo-membrane formed in enteritis caused by immune suppression due to the panleukopenia virus. To our knowledge, this is the first ultrasonographic description of a fibrinonecrotic cast and spontaneous passage of the colonic cast in the veterinary field.