The activity and selectivity of industrial cobalt- and iron-containing Fischer-Tropsch catalysts under pressure synthesis conditions in three-phase (slurry) and fixed-bed reactors were studied. The selectivity for hydrocarbons in the slurry process was shown to substantially depend on the temperature and the concentration of a finely dispersed catalyst in the liquid phase. The productivity of a Fischer-Tropsch catalyst under conditions of its contact with liquid hydrocarbons in the three-phase slurry system slightly changes in comparison with that of the fixed-bed process, but the hydrocarbon composition markedly changes toward increasing the proportion of gaseous hydrocarbons in the synthesis products.