Cot analysis ofChlorella DNA revealed that the genome of the unicellular green alga contained a small amount of repetitive sequences (at most 15% of the total DNA). Short repetitive sequences (SRS) of 170 bp produced by enzymatic digestion of algal DNA with eitherHaeIII,HinfI, orPstI, were found by polyacrylamide gel electrophoresis, and their copy number was estimated to be a few hundred (about 2% of the total repetitive sequences). All three members showed high sequence homology and could be be unified into one family, 'HaeIII family'. The family was divided further into two subfamilies,HinfI- (HaeIII-andHinfI-SRS) andPstI-(PstI-SRS) subfamilies, based on small sequence differences among the members. TheHaeIII family had characteristic structural features, including a considerable number of small unique sequence units (purine-CC) and both direct and inverted repeats, and were organized in tandem arrays in the genome.