The composition of ultraphytoplankton (<10 µm) sampled in surface waters (1 m depth) was investigated during summer 2010 in the Bay of Marseille (NW Mediterranean) using flow cytometry. In addition to groups of Synechococcus, Prochlorococcus, picoeukaryotes and nanoeukaryotes, an unidentified additional cluster was observed. The particles forming this cluster had a high abundance (>7 × 104 events mL−1), with both orange and red fluorescence intensities like Synechococcus, but with a size signal larger than that of 2 µm fluorospheres (beads). These unknown particles were sorted out by flow cytometry and then observed by epifluorescence microscopy: they appeared to be chain-forming microorganisms, just like trichomes of some diazotroph cyanobacteria with one heterocyst, but not unambiguously distal. The chains observed after cell sorting were not straight, but rather folded. To our knowledge, this is the first report of a possible bloom of such free trichomes. The small sample volume available for the delayed analysis and the limited resolution of our photomicrography did not allow for species identification. The brief occurrence of free trichomes in the Bay of Marseille with such a high abundance remains to be explained. High temperature in summer, induced stratification of the water column and nitrate depletion may be related to this event. High-frequency surveys of ultraphytoplankton assemblages at the single cell level appear to be necessary to observe such phenomenon more efficiently and document their dynamics.