The rabbit κ-casein cDNA was cloned and sequenced. One of the isolated clones included almost the entire 5′ end, while another clone corresponded to the 3′ end of the cDNA. No polyadenylation site was found and therefore this clone did not harbour the complete cDNA. The amino acid sequence of a full-length protein was deduced from the nucleotide sequence obtained for this partial cDNA. It revealed the presence of a chymosin cleavage site and five potential phosphorylation sites. Rabbit κ-casein was compared with those already described in other species. The rabbit sequence is closer to the ovine than to the mouse sequence. This result supports the idea that Lagomorpha are not closer to Rodentia than to Artiodactyla. The cDNA described above was used to study κ-casein gene expression in the rabbit mammary gland. This expression was induced primarily by prolactin in mammary gland organoids and was similar to αs1-casein gene expression in vivo. The κ-casein gene present in the casein gene locus is thus subject to the same regulation as the αs1-casein gene, although it has evolved from a fibrinogen gene.