DNA methylation controls human development and the Dnmt3a DNA methyltransferase is an essentail enzyme. Dnmt3a has been reported to multimerize on DNA and methylate DNA processively, which is contradicting. We show in this J. Biol. Chem. paper that multimerization of Dnmt3a on DNA increases its activity, but processive DNA methylation is not detectable. We conclude that Dnmt3a forms enzyme/DNA fibers. Our results help to understand the mechanism of DNA methylation and the effects of Dnmt3a somatic cancer mutations, which carry mutations at residues involved in its DNA interaction and multimerisation.