ABSTRACT The Aspergillus nimAgene encodes a Ser/Thr protein kinase which is required for mitosis, in addition to Cdc2, and which has been suggested to have a role in chromosomal condensation. In this study, we isolated a potential murine homologue of nimA, Nek2, which was shown to be expressed most abundantly in the testis of the adult tissues examined. Its expression in the testis was restricted to the germ cells, with highest levels detected in spermatocytes at pachytene and diplotene stages. Immunohistochemical analysis revealed that Nek2 localized to nuclei, exhibiting a nonuniform distribution within the nucleus. Nek2 appeared to be associated with meiotic chromosomes, an association that was better defined by immunolocalization to hypotonically dispersed meiotic chromosomes. This localization was more apparent in regions of dense chromatin, including the sex vesicle, and was also obvious at some of the chromosome ends. The presence of Nek2 protein was not unique to male germ cells, as it was found in meiotic pachytene stage oocytes as well. Furthermore, in an in vitro experimental setting in which meiotic chromosome condensation was induced with okadaic acid, a concomitant induction of Nek2 kinase activity was observed. The expression of Nek2 in meiotic prophase is consistent with the hypothesis that in vivo, Nek2 is involved in the G2/M phase transition of the cell cycle. Our results further provide evidence that in vivo, mouse Nek2 is involved in events of meiosis, including but not limited to chromosomal condensation.