The dinoflagellates are a group of diverse eukaryotic algae possessing a number of unique cellular properties. Evidence is presented for the presence of six capped small nuclear RNAs in these dinoflagellates. By several criteria such as the (a) presence of trimethylguanosine cap structure in U1 to U5 RNAs, (b) sequence homology between rat and dinoflagellate U2, U5 and U6 RNAs, (c) presence of other post-transcriptional modifications such as sugar and base modifications, and (d) association of Sm antigen with five of these six RNAs, the six RNAs of dinoflagellates appear to be similar to the well characterized U1 to U6 RNAs found in higher eukaryotes. This is the first demonstration of antigenic small nuclear RNA-containing particles in any unicellular organism. These results suggest that the U1 to U6 RNAs and the associated Sm antigen evolved at a very early stage of eukaryotic evolution. With respect to U small nuclear RNAs and their associated proteins, the dinoflagellates appear to exhibit eukaryotic characteristics.