Fifteen dogs which were serologically negative to the IFA test against Encephalitozoon developed antibodies in their sera following experimental infection. Six other dogs which were not tested prior to infection had titres to Encephalitozoon after experimental infection. In a sequential study in four dogs, antibodies first appeared between 32 and 39 days after infection. These results show that the IFA test would be suitable for epidemiological studies. Fifty serum samples collected from two kennels in which encephalitozoonosis had been confirmed showed an IFA test positive prevalence rate of 70%. In 220 serum samples submitted for various clinial pathological examinations the prevalence rate of antibodies to Ebcephalitozoon was 18%.