The cyst - theca relationship of Protoperidinium americanum (Gran & Braarud) Balech was investigated using single cyst germination techniques. This Protoperidinium has an unusual theca with four intercalary plates (plate formula 4′, 4a, 7″. 4c, 7s, 5″, 2″″). It has a distinctive brown capsulate cyst which has an apical archeopyle. The position of this species in the genus Protoperidinium is discussed. The distribution of the cyst on the west coast of Scotland is described.