The cloning and characterization of Ts-p41, an EF-hand calcium-binding protein of the protozoan parasite Tritrichomonas suis is describedNucleotide sequence data reported in this paper are available in the GenBankTM, EMBL and DDBJ databases under the accession numbers TSU011991 and TSU011992.. A T. suis cDNA library was screened with monospecific antibodies affinity purified on an immunoreactive 41kDa antigen in a Triton X-114 membrane-protein fraction. The resulting cDNA fragments turned out to be derived from 2 different genes encoding closely related Ts-p41 variants. The deduced amino acid sequences contained 6 EF-hand domains perfectly matching the canonical consensus motif and a putative C-terminal prenylation site. Northern and Southern hybridizations revealed that Ts-p41 was highly expressed and encoded by a gene-family. A cDNA encoding Ts-p41 was expressed as recombinant protein in Escherichia coli. By overlay with 45Ca it was demonstrated that the native and recombinant Ts-p41 proteins bind Ca2+. In immunofluorescence, epitopes recognized by anti-Ts-p41 antibodies were distributed as well on the anterior flagella as on the recurrent flagellum of the parasite. Our findings with the parabasalid T. suis suggest that multiple EF-hand bearing calcium-binding proteins might be a common phenomenon associated with flagellar motility.