Thirty-eight genes that encode bombyxin, the insulin-related brain secretory peptide of the silkmoth Bombyx mori, have been cloned and characterized. These genes have been classified into four families, A, B, C and D, according to their sequence similarity. All the bombyxin genes lack introns. Five of them have structural features of pseudogenes. The 38 genes cluster in the three DNA segments of Bombyx in unique distribution patterns. Their arrangement has been classified into three categories: gene pairs, gene triplets and single genes. In the pairs, two bombyxin genes belonging to families B and A (B/A) or to families B and C (B/C) are apposed with opposite transcriptional orientation. All triplets are arranged in the order of the family-B, family-C and family-A genes, and the transcriptional directions of the family-C and family-A genes are opposite to the direction of the family-B gene. The bombyxin gene triplets may have been generated by an unequal crossing-over between two gene pairs, B/A and B/C. Crossing-over may have occurred in the bombyxin family-B genes to increase their structural diversity. Duplications may have served to multiply the bombyxin gene triplets. These genomic rearrangements are thought to have led to the generation of multiple bombyxin gene copies and their diversity in structure and genomic organization.