The aim of this work was to compare the abilities of Rhizopus nigricans and Rhizopus oryzae cells to produce fumaric acid in free and immobilized cultures in a polyurethane foam. The cultivations were conducted in conical flasks in a rotary shaker as well as in a fermentor (7.5 L of volume). Apple juice containing 5% of reducing sugar was used as a source of carbon for fumaric acid biosynthesis and 5% of pure glucose. The immobilised cells of R. nigricans produced about 1.8%-2.5% of fumaric acid in the presence of glucose, and that was about 2-6-fold higher when compared with the free cells. About 2.2%-2.9% of fumaric acid were determined in the culture filtrates of R. oryzae and these values did not differ both for the free and immobilised cells because the free cells showed a pattern of self-immobilisation. Experiments conducted under the same conditions using apple juice biosynthesis of fumaric acid gave an average of 2.8% after 6 days of cultivation by the examined strains, and after changing the medium, the concentration of the acid increased to an average of 3.2%, which was about 28% higher than in the glucose-containing medium.