1. An electrophoretic method is described which resolves two groups of aconitase isozymes in human tissues, one group corresponding to the mitochondrial and the other to the soluble enzyme. This method has been used to screen human populations for electrophoretic variation. 2. Variant phenotypes of both the mitochondrial and the soluble aconitase are described, and family studies and sib-pair data demonstrate that the variation is genetically determined. 3. The variant isozyme patterns are accounted for in terms of allelic variation at two independent gene loci, ACONM and ACONS, which encode the mitochondrial and the soluble aconitase respectively. The electrophoretic survey has so far revealed two alleles at ACONM and seven alleles at ACONS. 4. The electrophoretic patterns shown by heterozygous individuals indicate that both soluble and mitochondrial aconitase are monomeric. The variant patterns also indicate that certain minor aconitase components resolved under the electrophoretic conditions employed represent 'secondary' isozymes of the primary translation products. 5. Population data on Europenas and Nigerians are presented and it is shown that both rare and common electrophoretic variants are segregating in the human species. The variation due to the three most common ACONS alleles in the Nigerians constitues a polymorphism.