1. Very-lysine-rich histone of a mammal (calf thymus), two sea urchins (Arbacia lixula and Paracentrotus lividus) and a sea cucumber (Holothuria tubulosa) were compared by using immunological techniques. 2. The degree of antigenic variability among all these histones was measured by quantitative microcomplement fixation. An evoluntionary diagram for this group of histones has been established. 3. The histone from sea cucumber is placed in an intermediate situation between those for calf thymus and sea urchins, and the immunological distance between the histones of the two urchins is very short.