Specific antibodies directed against the two yeast type-1 casein kinases (CK1) were used to study the localization of both 45 kDa and 27 kDa casein kinase species in yeast cells by immunofluorescence. Our results indicate that the larger and smaller CK1 species are localised in different compartments of the yeast cell. The 45 kDa enzyme is present in the cytoplasm of the cell both during the logarithmic and stationary growth phase. The 27 kDa CK1 was found in the nucleus in the cells in logarithmic growth phase while the enzyme from the stationary phase was present in the cytoplasm. Our results suggest that the 27 kDa casein kinase may play some role in yeast cell division control by displacement from the nucleus to the cytoplasm.