Unfertilized eggs of Oryzias latipes were artificially inseminated and incubated at 26+/-1 degrees C. Careful observation of the process of embryonic development by light microscopy allowed division of the process into 39 stages based on diagnostic features of the developing embryos. The principal diagnostic features are the number and size of blastomeres, form of the blastoderm, extent of epiboly, development of the central nervous system, number and form of somites, optic and otic development, development of the notochord, heart development, blood circulation, the size and movement of the body, development of the tail, membranous fin (fin fold) development, and development of such viscera as the liver, gallbladder, gut tube, spleen and swim (air) bladder. After hatching, development of the larvae (fry) and young can be divided into six stages based on such diagnostic features as the fins, scales and secondary sexual characteristics.