1. The genome of Amphioxus was investigated by DNA reassociation techniques for the amount of repetitive and non-repetitive sequences and its pattern of organization. 2. A comparison of the amount of non-repetitive DNA between Amphioxus and the tunicate Ciona intestinalis does not support the hypothesis that the Cephalochordates have arisen from the Tunicates by polyploidy. 3. In the Amphioxus genome repetitive and non-repetitive elements are predominantly arranged in a short period interspersion pattern. Conclusions are presented as to the evolution of contrasting genome organization patterns among vertebrates.