Blood films from a marine fish, the Cape Dory (Zeus capensis), trawled at depths between 149 and 389 m off the south and west coasts of South Africa, contained a new species of haemogregarine, Desseria zei sp. nov. The apicomplexan, found in 25/97 fish from the two regions, was generally intra-erythrocytic. Parasitaemia was usually low, but attained infection levels of 1/10 erythrocytes in one fish. The parasite stages were all gamonts and monomorphic, but likely varied in maturity; they were 3.3–4.5 μm wide by 14.7–18.3 μm long, curved with bluntly pointed ends, and existed singly within erythrocytes, or in pairs in the heavy infection. Their cytoplasm was deep blue, granular, and stained deeply at the anterior and sometimes, posterior extremities. Two or three vacuoles occurred in the cytoplasm between the anterior cap and the gamont nucleus, which was positioned in the posterior third of the parasite body. Gamonts lay close to the host cell nucleus, often curved around it. Distortion of host cells and host nucleus displacement occurred when single gamonts curved away from the host nucleus, or if they were paired. A few extracellular gamonts were observed, occasionally associated with the remnants of the erythrocyte nucleus.