Serine/threonine kinase 33 (STK33/Stk33) is a recently discovered gene whose inferred amino acid sequence translation displays characters typical for a calcium/calmodulin dependent kinase (CAMK). In this study we analysed the STK33/Stk33 RNA and protein distribution and the localization of the protein. The STK33/Stk33 expression pattern resembles those of some related members of the CAMK group. STK33/Stk33 displays a nonubiquitous and, in most tissues, low level of expression. It is highly expressed in testis, particularly in cells from the spermatogenic epithelia. Moreover, significant expression is detected in lung epithelia, alveolar macrophages, horizontal cells in the retina and in embryonic organs such as heart, brain and spinal cord. A possible role of STK33/Stk33 in spermatogenesis and organ ontogenesis is discussed.