Several invertebrate species possess haemoglobins in which each polypeptide contains multiple haem-binding domains, possibly reflecting the fusion of multiple monomeric globin transcriptional units at the gene level. We have now analysed the transcript of such a polymeric globin gene from the brine shrimp Artemia, which expresses three polymeric haemoglobins, each of relative molecular mass 260,000 (Mr 260K). These are formed by the variable association of two different subunit types, alpha and beta (refs 2,3). Haemoglobins I and III are homodimers of alpha and beta subunit types, respectively, and haemoglobin II is a heterodimer (alpha beta). The individual globin chains are of similar size (Mr 130K), but the exact nature of the differences between the two subunit types is unclear. Analysis of complementary DNA clones encoding one of the subunits of the Artemia dimeric haemoglobin showed that the globin messenger RNA encodes nine myoglobin-like domains, connected by linking peptides. The residues in the linkers are characteristic of those found generally in such protein linkers, and include turn-promoting amino acids. Each domain also contains the conserved residues that are required for functional haem-binding, and from analysis of the sequences it was predicted that they all can adopt the classic myoglobin-like fold. Analysis of the derived amino-acid sequences indicated that the individual domains are duplicated monomers that fused to form the polymeric globin some 200 Myr ago. The fusion of multiple transcriptional units for the evolution of a polymeric globin gene may have been a general mechanism for the appearance of such polymeric haemoglobins in invertebrates.