Members of the dual specificity tyrosine phosphorylated and regulated kinase family (Dyrk) were shown to have a highly testis-abundant or testis-restricted expression pattern. Furthermore, for some members of the family an involvement in gene expression regulation by phosphorylating transcription factors has been shown. Since little is known about the complex regulation of germ cell differentiation in spermatogenesis, we analysed the possible involvement of Dyrk kinases in this process. ISH experiments showed specific distribution of Dyrk kinases mainly in postmeiotic germ cell. We identified Dyrk4 as a testis-specific kinase with a very restricted expression to stage VIII postmeiotic spermatids. In vitro and in vivo experiments proved the enzymatic activity and suggested the cytoplasmatic localisation of Dyrk4. Finally, analysis of a Dyrk4 deficient mouse line showed that Dyrk4 is dispensable for male fertility, hence suggesting a functional redundancy of some Dyrk isoforms during spermiogenesis.