The specific activity of DNA methyltransferase increases in the nuclei of Sphaerechinus granularis sea urchin embryos at increasing stages of development. The activity reaches maximal value at about 20 h of growth, when embryos are at the mesenchyme blastula stage, then abruptly decreases and is essentially zero at about 35 h of development, when embryos are at the early gastrula stage. Both the increase and the drop of the activity are faster than embryonic cell duplication indicating that the enzyme is under strict control during development and that, in the more advanced embryo, a mechanism is activated to specifically block its activity.