The haploid DNA content has been determined for 275 species of teleostean fishes. Chromosome numbers are known for 73 of these. DNA content ranges from 4.4 picograms down to a minimum of 0.4. The more specialized or evolutionarily advanced fishes usually have less DNA than the more generalized forms. There are, however, several exceptions which may represent fish groups that are now in the process of evolutionary radiation. Chromosome number and DNA content are correlated, indicating that increases in DNA may have occurred by some form of polyploidy. An increase in DNA, followed by a gradual decrease, which is associated with specialization, appears to have accompanied fish evolution.