This study deals with the formation of the metacercarial cysts of four microphallid trematodes, Maritrema subdolum, M. arenaria, Levinseniella brachysoma and Microphallus claviformis. The first observable cyst was present around Maritrema arenaria 18 h p.i. (post-infection). The other species had not developed a cyst by day 8 p.i. but their cysts were apparent by day 16 p.i. These were bi-layered and that of M. subdolum was thicker than those of L. brachysoma and Microphallus claviformis of the same age. The structure of older cysts varied substantially between the four species. Microphallus claviformis and Maritrema subdolum cysts were fully formed at 30 days p.i. Like those of M. arenaria they were bi-layered, the outer layer (up to 3 μm thick) being electron-dense and the inner one (up to 7 μm thick) being less electron-dense. The cysts of fully formed L. brachysoma metacercariae were much more complex, composed of four layers, one of which was divisible into three sub-layers. It was concluded that the outer cyst layer was the product of secretory granules which were previously identified in cercarial tegument. The inner, thicker layer was derived from several sources. These included small tegument vesicles produced over the entire surface of the metacercariae, larger fragments of tegument released from the anterio-ventral region and material liberated from the metacercarial excretory bladder. This heterogeneous material accumulated in the cyst lumen for some time before becoming polymerized to form the thick inner layer or layers of the metacercarial cysts.