The cell shape of African trypanosomes is determined by the presence of an extensive subpellicular microtubule cytoskeleton. Other possible functions of the cytoskeleton, such as providing a potential framework for signalling proteins transducing information from the intracellular and extracellular environment, have not yet been investigated in trypanosomes. In this study, we have identified a novel cytoskeleton-associated protein in Trypanosoma brucei. CAP5.5 is the first member of a new family of proteins in trypanosomes, characterised by their similarity to the catalytic region of calpain-type proteases. CAP5.5 is only expressed in procyclic, but not in bloodstream, trypanosomes. Furthermore, CAP5.5 has been shown to be both myristoylated and palmitoylated, suggesting a stable interaction with the cell membrane. A bioinformatics analysis of the trypanosome genome revealed a diverse family of calpain-related proteins with primary structures similar to CAP5.5, but of varying length. We suggest a nomenclature for this new family of proteins in T. brucei.