A 865-bp insertion was detected within the nuclear small subunit (SSU) rDNA in two isolates (intron(+)) of the freshwater ascomycete Pseudohalonectria lignicola by the polymerase chain reaction (PCR), The intron sequences from the two isolates were identical to each other, and the exon sequences from the two intron(+) isolates were identical to those in the intron(-) isolates in the PCR-amplified region of rDNA. Reverse transcription PCR (RT-PCR) indicated that the intron was absent in the mature rRNA. The intron sequence has all the characteristics of a group I intron, including four conserved sequence elements (P, Q, R, and S), the presence of a U at the 5' splice site of the exon, a G at the 3' splice site of the intron, a putative internal guiding sequence, and the sequence fit a secondary structure model for group I introns. Like most introns found in nuclear rDNA, this intron was located at a highly conserved region and was devoid of long open reading frames.