Quantitative microfluorometric evaluation indicated that, in Lophius piscatorius, the DNA content of large neurons of a cluster located at the boundary between the medulla oblongata and the spinal cord varied from 4C to more than 5000C. This finding must be considered exceptional for the nervous system of Vertebrates. DNA contents were correlated to both nuclear and animal size. Utilization of AT and GC specific fluorochromes showed that the increase in DNA content is due to differential genome amplification involving GC-rich DNA sequences.