We have cloned a gene, CRP65, from genomic DNA of Giardia duodenalis (Gd) which contains four 228-bp tandem repeat units between a short (48bp) 5' and long (942 bp) 3' non-repeat region. CRP65 encodes a Cys-rich protein (CRP) with the typical transmembrane domain and CXXC amino acid (aa) motif of Gd CRP. Comparison of the nucleotide (nt) and deduced aa sequences of CRP65 and a gene we cloned previously. CRP136, indicates that the genes are highly homologous in the entire non-repeat regions, but not in the repeat regions. The repeat unit of CRP65 was found to be homologous to epidermal growth factor (EGF)-like domains from different proteins. Analysis of Gd genomic DNA showed that there are multiple copies of CRP65 and each copy varies in the number of repeat units, as well as in certain restriction sites in the units. In Gd strain WB-1B, a 2.0-kb transcript encoded by the gene was expressed, while in a metronidazole-resistant line (WB1B-M3) induced from WB-1B, two longer transcripts (5.5 and 7 kb) were expressed. Based on our results, we suggest that there is a unique CRP family in the Gd genome, whose members, including CRP65 and CRP136, carry various repeat units within a highly conserved 'cassette'. CRP65 may be involved in EGF-like interactions with the host proteins.