To describe the early phenotype of girls with full mutation fragile X, we used 54 observations of 15 girls between the ages of 6 months and 9 years to examine developmental trajectories as measured by the Battelle Development Inventory. In this sample, autistic behavior was associated with poorer developmental outcomes, primarily due to interactions of age with autistic behavior, even though autistic behavior, measured continuously, was relatively mild. Although this small sample, ascertained primarily through male relatives with fragile X syndrome, limits generalizability, considerable variability in developmental outcome in young girls was documented. In addition, findings support previous research suggesting that even mild autistic behaviors in girls can be associated with developmental outcomes.