We have isolated the Schizosaccharomyces pombe orthologue of the Saccharomyces cerevisiae MOB1 gene in a screen designed to enrich for septation mutants. The gene is essential, and cells lacking it display a phenotype typical of septation signalling network mutants. mob1p is located on both spindle pole bodies throughout mitosis. In addition it is also co-localised with the medial ring later in mitosis, and flanks the septum as the medial ring contracts. We also demonstrate that mob1p can be precipitated from cells in a complex with the septation regulating kinase sid2p.