The apicomplexan parasite Cryptosporidium parvum invades and multiplies primarily in the brush border cells of the intestinal mucosa causing in AIDS patients a severe diarrhoea that represents a significant contributing factor leading to death. Morphological analysis indicates that the invasion machinery of C. parvum is similar to the apical complex of other parasites of the phylum Apicomplexa. We provide here evidence indicating that C. parvum also shares with these parasites a molecule crucial for the invasion of host cells. We have cloned a 3894 bp-long C. parvum cDNA encoding a protein characterised by sequence and structural similarities with members of the thrombospondin (TSP) family previously described in apicomplexan parasites of the genera Toxoplasma, Eimeria and Plasmodium. This novel C. partum molecule, the TSP-related adhesive protein of Cryptosporidium-1 (TRAP-C1), is encoded by a single copy gene containing no introns. TRAP-C1 is localised in the apical end of C. parvum sporozoites and is structurally related to the micronemal proteins MIC2 of Toxoplasma and Etp100 of Eimeria, which are involved in host-cell attachment and/or invasion. The identification of TRAP-C1 sheds new light on the molecules possibly involved in the invasion process of intestinal cells by C. parvum. We have also analysed the sequence variation of TRAP-C1 among C. parvum isolates and in the closely related species C. wrairi.