We report here on the characterisation of Ulp1, a component of the SUMO modification process in S. pombe. Recombinant S. pombe Ulp1 has de-sumoylating activity; it is involved in the processing of Pmt3 (S. pombe SUMO) and can, to a limited extent, remove Pmt3 from modified targets in S. pombe cell extracts. ulp1 is not essential for cell viability, but cells lacking the gene display severe cell and nuclear abnormalities. ulp1-null (ulp1.d) cells are sensitive to ultraviolet radiation in a manner similar to rad31.d and hus5.62, which have mutations in one subunit of the activator and the conjugator for the ubiquitin-like protein SUMO respectively. However ulp1.d cells are less sensitive to ionising radiation and hydroxyurea (HU) than are rad31.d and hus5.62. ulp1-null cells are defective in processing precursor Pmt3 and display reduced levels of Pmt3 conjugates compared with wild-type cells. The slow growth phenotype of ulp1 null cells is not substantially rescued by over-expression of the mature form of Pmt3 (Pmt3-GG), suggesting that the de-conjugating activity of Ulp1 is required for normal cell cycle progression. During the S and G2 phases of the cell cycle the Ulp1 protein is localised to the nuclear periphery. However, during mitosis the pattern of staining alters, and during anaphase, Ulp1 is observed within the nucleus. Ulp1 localisation at the nuclear periphery is generally re-established by the time of septation (S phase).