In the brains of four species of cyclostomes (two species each of lampreys and hagfishes), immunoreactive calcitonin-producing cells (iCT cells) were located immunohistochemically by the peroxidase-antiperoxidase method using anti-salmon calcitonin antiserum. In the case of both the adults and the ammocoetes of the brook lamprey (Lampetra reissneri) which lives in freshwater throughout its life, iCT cells were found in two distinct areas: in the pars ventralis hypothalami of the diencephalon and in the torus semicircularis of the mesencephalon. The iCT cells in the diencephalon are classified as bipolar nerve cells, and those in the mesencephalon are classified as multipolar nerve cells. In both the anadromous and catadromous arctic lamprey (Lampetra japonica), iCT cells were present only in the diencephalon, and those were bipolar nerve cells. There seemed to be no differences in the numbers and the immunostainability of the iCT cells, despite the different environments inhabited by the lampreys. In the hagfishes (Eptatretus burgeir and Paramyxine atami) that inhabit seawater throughout their lives, iCT cells were also found only in the diencephalon, although they were very few in number and exhibited poor immunostainability.