First studies of the microalga flux over the Lomonosov Ridge in the northern Laptev Sea were carried out with a sediment trap at the yearlong mooring station LOMO-2, installed at a 150-m depth (81°04.3′ N; 138°55.2′ E, sea depth 1703 m) from September 15, 1995 to August 16, 1996. These studies demonstrated essential seasonal variation of the vertical microalga flux. It was shown that, in summer, a diverse flora (composed mainly of cryophylic diatoms) grows intensively beneath the permanent ice cover. Strongly pronounced seasonal variations of the microalga growth correlate closely with the solar radiation. Exactly during the maximum insolation period, from the middle of July until the end of September, the microalga flux is hundreds of times higher than that in the rest of the year. The summer values of the microalga flux over the Lomonosov Ridge in the northern Laptev Sea are similar to those in the Weddell Sea in the Antarctic and exceed the summer flux values in the Norwegian and Greenland Seas and in the St. Anna Trough (the northwestern Kara Sea).