Multiple electrophoretic forms of larval haemoglobin have been found by studying a Sardinian species of Chironomus. Studies of molecular size, carried out by gel filtration and electrophoresis, have suggested the exclusive occurrence of haemoglobin monomeric forms, the molecular weight of which has been plotted in a range of values of 13,000-15,000. This finding supports the hypothesis stating that each electrophoretic variant is the product of a single structural gene.