mRNA from two molluscs, the snail Levantina hierosolima and the cuttlefish Sepia officinalis, and one arthropod, the scorpion Leiurus quinquestriatus, were found to contain species that hybridize with an oligodeoxynucleotide sequence corresponding to the six-amino-acid sequence His-His-Trp-His-Trp-His postulated to carry binuclear copper in arthropod haemocyanins. The duplexes formed between the oligodeoxynucleotide and mRNA from Leiurus and Levantina had similar 'melting' temperatures, namely 46.5 and 44.5 degrees C respectively. The hybridizable mRNA species were identified with mRNA that codes for haemocyanin, Hc mRNA. Electrophoresis under denaturing conditions of Leiurus Hc mRNA gave a single band corresponding to 2.3 kb (kilobases), consistent with the value of 75 000 reported for the Mr of the polypeptide chain of arthropod haemocyanin. Electrophoresis of mollusc Hc mRNA yielded RNA bands corresponding to 9.4, 6.7, 4.0 and 1.7 kb for Levantina and 9.5, 2.8 and 1.7 kb for Sepia. The 9.4 and 9.5 kb species represent authentic Hc mRNA and are consistent with an Mr of 350 000 for molluscan haemocyanin polypeptide chain. The faster-moving RNA bands are attributed to Hc mRNA cleavage by nucleases during isolation of mRNA. Analysis of the electrophoretic band pattern indicates a multi-unit structure for mollusc Hc mRNA.