F-box proteins serve as mediators in targeting bound target proteins for ubiquitination and destruction. We here describe the roles of two F-box proteins, CFB1 and CFB2, in the trypanosome cell cycle. Five almost identical copies of CFB1 are arranged in a direct tandem repeat on Trypanosoma brucei chromosome 1; immediately downstream is a single CFB2 gene. RNAi targeting CFB1 in bloodstream-form trypanosomes had a transient effect on growth and mitosis. Depletion of CFB2, in contrast, resulted in immediate growth arrest and rapid cell death. CFB2-depleted cells accumulated nuclei and kinetoplasts with the corresponding numbers of basal bodies and flagella. The CFB2 transcript was less abundant in procyclic-form trypanosomes, and RNAi against CFB2 in these forms had no effect on growth. These results suggest that CFB2 is required for bloodstream-form trypanosome cytokinesis.