In Schizosaccharomyces pombe the onset of mitosis is regulated by a network of protein kinases and phosphatases. The M-phase inducing Cdc2-Cdc13 cyclin-dependent kinase is inhibited by Weel tyrosine kinase and activated by Cdc25 phosphatase. Weel is negatively regulated by Niml protein kinase. Here, we describe investigations aimed at better understanding the role of Niml in the mitotic control. The most important finding to emerge from these studies is that Weel and Niml have different patterns of intracellular localization. Immunofluorescence confocal microscopy has revealed that Niml is localized in the cytoplasm, whereas its substrate Weel is predominantly localized in the nucleus. Previous studies showed that the Cdc2-Cdc13 complex is located in the nucleus. Diversion of Niml to the nucleus, accomplished by addition of the SV40 nuclear localization signal, caused the advancement of M, confirming that Niml has restricted access to Weel in vivo. We propose that the intracellular distribution of Niml and Weel may serve to coordinate the regulation of nuclear Cdc2-Cdc13 with cytoplasmic growth.