Glutamine synthetase was shown to be mitochondrial in liver tissue and cytosolic in brain tissue of the holocephalan Hydrolagus colliei. The liver mitochondrial form has an Mr approximately 1,500 larger than the brain cytosolic form, so the two represent compartmental isozymes. Similar isozymes have been shown to be present in elasmobranchs, and their occurrence in holocephalans suggests the two groups share a common ancestry.