Phosphoinositide-specific phospholipase C (PLC) is a key enzyme in phosphoinositide turnover and is involved in a variety of physiological functions. We analyzed PLCdelta1 knockout mice and found that PLCdelta1 is required for the maintenance of skin homeostasis. However, there were no remarkable abnormalities except hair loss and runting in PLCdelta1 knockout mice, even though PLCdelta1 is broadly distributed. Here, we report that mice lacking both PLCdelta1 and PLCdelta3 died at embryonic day 11.5 (E11.5) to E13.5. PLCdelta1/PLCdelta3 double-knockout mice exhibited severe disruption of the normal labyrinth architecture in the placenta and decreased placental vascularization, as well as abnormal proliferation and apoptosis of trophoblasts in the labyrinth area. Furthermore, PLCdelta1/PLCdelta3 double-knockout embryos supplied with a normal placenta by the tetraploid aggregation method survived beyond E14.5, clearly indicating that the embryonic lethality is caused by a defect in trophoblasts. On the basis of these results, we conclude that PLCdelta1 and PLCdelta3 are essential in trophoblasts for placental development.