Phosphoinositide (PI) signal is based on the metabolic chain of membrane phospholipids and it is thus ubiquitous in eukaryotic cells. In PI signal, diacylglycerol (DAG) is the first identified second messenger to enhance the activity of protein kinase C (PKC) and it is converted to phosphatidate (PA) by DAG kinase (DGK). Therefore, DGK is a key regulator of PKC activity by attenuation of DAG. The present study was attempted to examine the localization of DGK in liver fluke by immunohistochemistry analysis as well as conventional transmission electron microscopy (TEM). Distinct immunoreactivity for rat DGKg was detected in Mehlis gland in contrast to actual absence of the immunoreactivity throughout the worm. The immunostained profiles appeared in forms of radiation from central structural elements of numerous slender ones. In immuno-DAB (diaminobenzidine) TEM, the radiating immunostained structures were revealed to represent the epithelial cells of Mehlis gland. It is necessary to examine in Western blotting whether the molecular size of this protein corresponds to DGK and its cDNA cloning remains to be elucidated. Considering the plausible function of Mehlis gland such as (a) lubrication of the uterus for the passage of eggs, (b) activation of spermatozoa, (c) release of shell globules from the vitelline cells, the present finding suggests that DGK is a potential target to inhibit the proliferation of liver fluke, leading to a useful way to protect the disease.