Dinophysis species may contain Diarrhetic Shellfish Toxins (DST). They occur all along the Norwegian coast and may in some regions prevent harvesting of mussels several months each year. The content of toxins seems to vary considerably between and within species. Also the morphology is very variable within some species and the species-delineation can at times be unclear. We have examined the phylogenetic relationship of four Dinophysis/ Phalacroma species (D. acuminata,D. acuta, D. norvegica and P. rotundatum) isolated from Norwegian waters inferred from the 18S ribosomal RNAgene. The genetic variability within the species D.acuminata and D. norvegica was examined by analysing the first internal transcribed spacer (ITS1) in 5 isolates per species collected at different times of the year and from two localities off the coast of southern Norway. The three photosynthetic species D. acuta, D. acuminata, and D.norvegica were very similar within the 18S rRNA gene and differed in only 5-8 out of 1802 bp. The non-photosynthetic P.rotundatum, however, differed in ca. 60 bp compared to the three photosynthetic species. This supports the original distinction between Dinophysis and Phalacroma. In the phylogenetic analyses the Dinophysis/ Phalacroma (dinophysoid) species fall into a common clade that are associated to a branch composed of gymnodinioid, prorocentroid and peridinioid species (GPP complex). Despite differences in morphology between the five isolates of D. acuminata , they all had almost identical ITS1 sequences. Similarly, all 5 isolates of D.norvegica were identical in this non-coding region. The ITS1-sequences in D. acuminata and D. norvegica were very similar, differing in << 10 bp. Further work includes development of oligonucleotide probes based on sequence differences within the rRNA operon.