During development, Schwann cell numbers are precisely adjusted to match the number of axons. It is essentially unknown which growth factors or receptors carry out this important controlin vivo. Here, we tested whether the type II transforming growth factor (TGF) β receptor has a role in this process. We generated a conditional knock-out mouse in which the type II TGFβ receptor is specifically ablated only in Schwann cells. Inactivation of the receptor, evident at least from embryonic day 18, resulted in suppressed Schwann cell death in normally developing and injured nerves. Notably, the mutants also showed a strong reduction in Schwann cell proliferation. Consequently, Schwann cell numbers in wild-type and mutant nerves remained similar. Lack of TGFβ signaling did not appear to affect other processes in which TGFβ had been implicated previously, including myelination and response of adult nerves to injury. This is the firstin vivoevidence for a growth factor receptor involved in promoting Schwann cell division during development and the first genetic evidence for a receptor that controls normal developmental Schwann cell death.