Thelohanellus hovorkai was designated as “hemorrhages thelohanellus” because of severe haemorrhages on the body surface of the affected common carp. The diagnosis of this disease is related to the detection of spores, whose size varies, according to researches, in fish from open water and carp fish raised in aquaculture. The disease is manifested by disturbances in the central nervous system, anemia and absence of progress in fingerlings, where numerous spores are diagnosed in the tissues of fish. The investigations were conducted in the northern Serbia in 15 fish ponds during the period 2007-2009. The methods that have been used were clinical observations, light microscopy and classical pathohistology with H&E stain. The presence of Thelohanellus hovorkai was detected in common carp fingerlings older than 90 days and female carp fish. Effective therapeutic measures are not known but the general preventive measures related to the preparation of facilities for the cultivation of carp fingerlings give favorable results.