PP5 is a member of the PPP family of serine/threonine protein phosphatases and is present in all eukaryotes. We previously cloned and characterized a PP5 homologue from Trypanosoma brucei. Here, we synchronized the T. brucei procyclic form by hydroxyurea treatment and showed that TbPP5 expression is regulated during cell cycle progression. TbPP5 transcript and protein levels were maximal in the G1 phase of the cell cycle, and reduced about 3-fold in the G2/M phase. To further evaluate its function, TbPP5 expression was depleted in both procyclic and bloodstream forms of T. brucei by RNA interference. In the procyclic form, TbPP5 knockdown resulted in a moderate reduction in cell growth. However, in the bloodstream form, ablation of TbPP5 caused an 8-fold decrease in cell growth. Furthermore, TbPP5 overexpression conferred the ability of procyclic cells to grow in serum-deprived conditions suggesting that TbPP5 acts downstream of serum factor–induced growth in T. brucei. Taken together; these findings suggest that a serum factor (or factors) induces up-regulation of TbPP5 expression during the G1 phase, which is required for proper cell growth.