Development stages of a haemogregarine were found in Giemsa-stained heart blood smears of 3 of the 4 horned blennies (Parablennius cornutus) captured at De Hoop Nature Reserve, South Africa. Gamonts of this haemogregarine conformed to an existing description of Desseria (Haemogregarina) fragilis from P. cornutus, but intraerythrocytic trophozoites, as well as meronts undergoing division, were reported for the first time for this species. A detailed redescription of D. fragilis allowed its taxonomic re-evaluation. Intraerythrocytic division excluded D. fragilis from Desseria, whereas morphometric similarities and identical patterns of development lead to the conclusion that D. fragilis is the cosmopolitan haemogregarine, Haemogregarina bigemina. Nomenclatural correction synonymizing the 2 species is recommended.