Karyolysus is a genus of coccidia. With the exception of K. sonomae whose vertebrate host is the yellow-legged frog (Rana boylii boylii), species in this genus only infect lizards of the genus Lacerta.

The genus was created by Labbe in 1894 for those species of sporozoans that fragmented the host nucleus. Reichenow in 1921 established that this property was of dubious taxonomic value. He proposed a different set of criteria for inclusion in this genus and these are the one currently in use.

The type species is Karyolysus lacertae .

Schizogeny occurs in the endothelial cells

Gametogony occurs in the erythrocytes after their penetration by the merozoites.

Differentiation from the genus Haemogregarina may be difficult as in both genera gametocytogenesis occurs in the erythrocytes and have similar morphology. They can be distinguished on the location of schizogony: In Karyolysus this occurs in the endothelial cells of the visera while in Haemogregarina schizogony occurs in the bone marrow.

Starting with infected mites which the host lizard ingests the veriform sporozoites are released from the sporocysts. These newly liberated sporozoites bore into the intestinal wall and enter the lymphatic vessels and the blood stream. On reaching the viscera and in particular the liver the sporozoites penetrate the endothelial cells. There they form ovoid schizonts within a membranous capsule. They remain in these cells for some time and give rise to 8 to 30 merozoites. The merozoites remain in the cell for some time before rupturing it and entering the blood stream.

On entering the blood stream the merozoites penetrate other endothelial cells. There they grow and undergo schizogeny producing a number of small merozoites. These second generation schizonts are smaller than the first generation and lack a discernable nucleus. These are also eventually released into the blood stream, this occurs usually day 42 post infection.