The presence of extrachromosomal nuclear ribosomal DNA (rDNA) in the unicellular alga Euglena gracilis has been established. This rDNA is circular. Each circle is 3.8 micron long and contains one rDNA unit. Oligomers are rare. Extrachromosomal rDNA is present in large amounts during the exponential phase of growth and appears less abundant during the stationary phase. It was found in all wild-type and mutant strains of Euglena examined. Our estimations suggest that rDNA in Euglena is mainly extrachromosomal. Research of extrachromosomal rDNA in spinach and Petunia was negative.