Merocystis is a genus in the phylum Apicomplexa.

This genus was created in 1911 by Dakin He discovered this parasite - Merocystis kathae - in the renal organ of the whelk (Buccinum undatum). Its life cycle has been studied in some detail by other workers.

A second species was described - Merocystis tellinovum - by Buchanan in 1979 but this has since been moved to the genus Pseudoklossia.

There is one species known in this genus.

The life cycle follows a seasonal course. The earliest stages may appear in the kidney any time between March and June. Growth proceeds and sexual differentiation takes place but mature gametes are not found until November, when the first stages of sporogony are first found. Mature spores containing sporozoites are formed in January and become increasingly common up to May. Degenerating stages of spores are common in summer.

The earliest stages within the renal organ are 10 μm in diameter.

To the naked eye the later stages of the parasite have the appearance of a rounded milky white dot.

The trophozoites appear opaque and occasionally granular.