GROWTH factors synthesized by ovarian somatic cells directly affect oocyte growth and function(1-6), but it is unclear whether oocyte-secreted factors(6-9) play a reciprocal role in modulating somatic cell functions in vivo. During the functional analysis of members of the transforming growth factor-beta superfamily in mouse development(10-15), we have uncovered a new family member, growth differentiation factor-9 (GDF-9), which is required for ovarian folliculogenesis. GDF-9 messenger RNA is synthesized only in the oocyte from the primary one-layer follicle stage until after ovulation(16,17). Here we analyse ovaries from GDF-9-deficient female mice and demonstrate that primordial and primary one-layer follicles can be formed, but there is a block in follicular development beyond the primary one-layer follicle stage which leads to complete infertility. Oocyte growth and zona pellucida formation proceed normally, but other aspects of oocyte differentiation are compromised. Thus, GDF-9 is the first oocyte-derived growth factor required for somatic cell function in vivo.