The Dinoflagellates are an intriguing and diverse protist group in terms of morphology, ecology and geographical distribution amongst others. The proportion of heterotrophic and photosynthetic taxa are rougly even. However, many of the photosynthetic taxa are actually mixotrophic. Together with the diatoms and haptophytes dinoflagellates vastly contributes to the primary production in marine environments. Besides this positive feature, many dinoflagellate taxa can produce potent toxins leading to sea-food borne dieseases and fish kills when they occur in high concentration. Given the importance of dinoflagellates they are vigorously studied from many aspects. Molecular biology and bioinformatics development are utilized by researchers of dinoflagellates. In this work diversity and seasonal occurence is investigated; yearly dynamics is discussed based on 12 selected OTU (Operational Taxonomic Unit). Samples were collected between September 2009 and June 2011 in Outer Oslofjorden, OF2 station. A single cast of CTD rosette with Nishkin bottles attached was done with monthly intervals. Fluorescence, nutrients (Nitrogen, Silica, Phosphorous), chlorophyll-a, as well as irradiance was measured on each occasion. Water samples obtained by Nishkin bottles from 1 m depth were filtered with 45 µm and 3 µm filters in order to get the nanoplankton size fraction. Then, the total ribosomal RNA of the SSU V4 region was extracted and converted to cDNA. The SSU V4 rDNA region was sequenced. The DNA was extracted from culture stocks of strains UiO306, UiO307 and UiO312 and the SSU and LSU rDNA regions were sequenced. Data from the 454-pyrosequencing was processed using Qiime. rDNA sequences of the cultured species were edited in BioEdit. RaxML phylogenetic tree was built to unravel their identity. In total 374 dinoflagellate OTUs were detected by the 454-pyrosequencing approach at station OF2 in Outer Oslofjorden during the period September 2009-June 2011. The most abundant taxa, based on read abundance, were Gymnodiniales and Peridiniales, which had representatives in all samples. Syndiniales, Dinophysiales and Gonyaulacales were also important components of the dinoflagellate communities. Strains UiO306 and UiO312 were identified as Pelagodinium béii, whereas UiO307 was Scrippsiella donghaiensis. Dinoflagellate diversity showed seasonal variations, being the most diverse in autumn and the lowest in the late winter/early spring. Both P. béii and S. donghaiensis were detected in Outer Oslofjorden thanks to the 454-pyrosequencing data. The applied methods have flaws, which are important to be aware of. The main source of errors of 454-pyrosequencing is homopolymers and the mistakes made during the PCR (single base errors). Collection of samples assumed heterogeneous phytoplankton distribution in the water column, whereas phytoplankton communities have rather patchy pattern. Scrippsiella donghaiensis occurred more frequently than P. béii in Outer Oslofjorden. Using more sampling occasions, including sequencing of additional depths and including more genetic information (LSU rDNA, ITS) would help to provide a more reliable and exhaustive description of the detected seasonality of dinoflagellate diversity and dynamics.