In the nuclear encoded small subunit ribosomal DNA (18S rDNA) of symbiotic alga ofParamecium bursaria(F36 collected in Japan) possesses three intron‐like insertions (Hoshina et al., unpubl. data, 2003). The present study confirmed these exact lengths and insertion sites by reverse transcription‐PCR. Two of them were inserted atEscherichia coli16S rRNA genic position 943 and 1512 that are frequent intron insertion positions, but another insertion position (nearly 1370) was the first finding. Their secondary structures suggested they belong to Group‐I intron; one belongs to subgroup IE, others belong to subgroup IC1. Similarity search indicated these introns are ancestral ones. © 2004 American Institute of Physics