We report the characterization of the 5' region of the gene coding for the 2 alpha fibrillar collagen chain of the sea urchin Paracentrotus lividus. This sequence analysis identified the intron/exon organization of the region of the gene coding for the signal peptide, the cysteine-rich domain and the 12 repeats of the four-cysteine module of the unusually long amino-propeptide. This still unknown four-cysteine motif is generally encoded by one exon, which confirms that the distinct amino-propeptide structures of the fibrillar collagens arise from the shuffling of several exon-encoding modules. Moreover, Southern-blot analysis of the sea urchin genome and sequencing of selected genomic clones allowed us to demonstrate that several sea urchin genes could potentially code for the four-cysteine module. Curiously, one of these genes lacks the exons coding for four repeats of this motif while, in another gene, the same exons are submitted to an alternative splicing event.