This paper describes the cloning of the ornithine decarboxylase gene from a red seaweed, Grateloupia imbricata (Rhodophyta), the characterization of its expression throughout the reproductive process, and demonstrates how polyamines are involved in seaweed reproduction. In addition, the data indicate that the basal perennial and non-spore-forming thalli behave physiologically and genetically differently from the distal reproductive tissue. The common polyamines putrescine, spermidine and spermine have been associated with carposporogenesis in red seaweeds. Ornithine decarboxylase (ODC, EC 4.1.1.17) produces the diamine putrescine from the non-protein amino acid, ornithine. ODC is predominant in the synthesis of polyamines in G. imbricata. The gene encoding the ornithine decarboxylase in G. imbricata was cloned by genomic polymerase chain reaction (PCR) using degenerate primers against conserved motives, followed by chromosome walking using inverse PCR (iPCR). The encoded protein (GiODC, accession # FJ223132) was very similar to other ODCs, bearing the characteristic conserved domain of pyridoxal-dependent decarboxylases. The expression of the GiODC gene was investigated by real-time PCR and in situ hybridization (ISH), and was observed to vary according to cystocarp differentiation. It was weakly transcribed in apical parts of fertile tissue where the cystocarps are located, while the transcript levels were comparatively high in the basal part. This expression pattern correlated with the levels of free polyamines, which were higher at the basal part.