Fumaric acid is a dicarboxylic acid used extensively in synthetic resins, food acidulants, and other applications, including oil field fluids and esters. The filamentous fungus Rhizopus oryzae is known for its ability to produce and accumulate high levels of fumaric acid under aerobic conditions. In this work, the overexpression of native fumarase encoded by fumR and its effect on fumaric acid production in R. oryzae were investigated. Three plasmids containing the endogenous fumR gene were constructed and used to transform R. oryzae, and all transformants showed significantly increased fumarase activity during both the seed culture (growth) and fermentation (fumaric acid production) stages. However, fumarase overexpression in R. oryzae yielded more malic acid, instead of fumaric acid, in the fermentation because the overexpressed fumarase also catalyzed the hydration of fumaric acid to malic acid. The results suggested that the overexpressed fumarase, encoded by fumR, by itself was not responsible for the over-production of fumaric acid in R. oryzae. (C) 2012 Elsevier Ltd. All rights reserved.