Live birth is a method of reproduction in which the embryo develops in the maternal organism, feeds directly from it, and is born as a fully formed calf. It is found in the European beluga. In some sharks and rays yolk sac embryo, equipped with blood vessels, after the use of yolk grows to the wall of the enlarged part of the oviduct, called the uterus, forming a kind of placenta. By diffusion and osmosis, nutrients from the blood vessels of the mother through the placenta penetrate into the blood vessels of the embryo, and metabolic products in the same way get from the embryo to the mother. The baby is born fully developed.

In contrast to fish that lay eggs, viviparous fish give birth to more or less formed fry. The largest number of viviparous fish among sharks (most sharks, stingrays, eagle rays, giant rays). Among bony fishes, live births are known in beluga (Zoarces viviparus), sea basses (Sebastes), Baikal cephalopod (Comephorus baicalensis), many carps (Anableps tetraphthalmus), some freshwater half-fishes (Hemirhamphidae) and others. In all viviparous fishes, fertilization is internal. Distinguish viviparous fish, in which the embryo is connected to the body of the mother and feeds on its account (eg, in the marten shark), and oviparous, in which the eggs are in the body of the mother, but not connected to it and develop at the expense of the yolk (most). Fry development occurs in the ovaries or modified oviducts and lasts from 2-3 weeks (in carpodontids) to several months (in sharks). Sharks give birth to one or several large (up to 70 cm long) fry, most bony - several dozens or hundreds; in the Murmansk sea bass - up to 350 thousand small larvae (up to 8 mm long). In aquariums breed viviparous fish of the family Poecilidae: guppies, swordtails, etc.