Most (1918bp) of the nucleotide sequence of the 18S ribosomal RNA gene of Heronimus mollis was determined. We aligned this sequence with 18S rRNA sequences from 7 other digeneans and an aspidobothrean. Using these data we explored the phylogenetic position of H. mollis. No support was found for the hypothesis that H. mollis is the sister taxon of the remaining digeneans examined here; similarly, no support was found for the hypothesis that H. mollis is closely related to the paramphistomes. These analysis together with the skewness of the frequency distribution of the lengths of 100,000 trees sampled randomly from the set of all possible trees (generated from these data) indicate that 18S rRNA sequences are valuable for phylogenetic inference in the Digenea.