Conjoined twins (polyembryonic fish) were observed in batches of blue tilapia Oreochromis aureus (Steindachner) and Nile tilapia O. niloticus (L.) embryos incubated at 28 °C. Heat shock (41.8 °C) treatment of fertilized eggs for 3–4 min applied between 4 min and 27 min after fertilization increased the twinning rate by three or four times over that of unshocked eggs. Two basic morphological variations – ventrally or laterally conjoined twins – were observed. Twins did not survive beyond 21 days after hatching. The proportion of twins in heat-shocked egg batches of both species combined ranged from 0.32% to 0.40%.