An electrophoretic study of ribosomal proteins isolated from embryonic tissues demonstrates striking differences when ribosomal proteins from embryos of distantphyla, as sea urchins and chickens, are compared. Some differences are also evident between those of close genera as Paracentrotus lividus and Sphaerechinus granularis. No differences were detected between the ribosomal proteins of different developmental stages of sea urchins or between different tissues of chick embryos.