The isozymes of malic dehydrogenase (MDH) of maize have been separated by starch gel zone electrophoresis. The MDH isozyme pattern was the same in different organs and developmental stages. The subcellular distribution of the different isozymes was established by means of differential centrifugation. The MDH isozymes were found to be differentially distributed in the cytoplasm, the mitochondria, and the glyoxysomes. The latter proved the only exception to the pattern constancy among organs insofar as they are present only in the scutella. Among several inbred maize strains examined, only variants of the mitochondrial isozymes were found. Crosses between strains with different isozyme patterns gave Mendelian inheritance: thus, these mitochondrial isozymes are under nuclear-gene control.