Influence exerted by various concentrations (0.01 to 50 ppm) of some chlorinated hydrocarbons (Aroclor 1254, Aroclor 1242, pp'DDE, pp'DDT and Lindane (ɣ НСН)) upon the photosynthetic assimilation of 14C02 in Antarctic marine diatom assemblage dominated by Corethron criophilum and some species of Nitzschia (Fragilariopsis group) has been investigated. The photosynthesis was fully inhibited by Lindane (ɣ HCH) in all applied concentrations To smaller extent the photosynthetic process was inhibited in turn by Aroclor 1242, pp'DDE and pp'DDT successively. Aroclor 1254 proved to be the least toxic. The possibility of the decrease of the primary production of the Antarctic diatoms caused by the chlorinated hydrocarbons was discussed.