It has long been evident that child development is subject to the pull of both biological and environmental forces. However recent secular trends in biological maturation have lowered the age of onset of the adolescent growth spurt to 9 for girls and 11 for boys. This critical biological trend alongside recent civil rights initiatives (e.g., Title IX) and the exponential growth in youth sports suggests a need to re-examine sources of variability in development of key fundamental movement skills. The purpose of this study was to examine developmental changes associated with mature throwing by children in grades k-8. Specifically, the study addressed the following research questions: 1) what are the characteristics of individual differences in throwing over time? And 2) are differences in developmental growth trajectories of mature throwing systematically related to age, sex, and/or sports participation? In keeping with the spirit of Schaie's General Developmental Model, we employed a sequential design that included cross-sectional, longitudinal, and time-lag components. This design allowed us to examine the influence of age, sex, and sport participation while reducing cohort effects. Following a pilot study to examine inter-rater reliability, we tested children on mature throwing using the Ohio State University Scale of Intra Gross Motor Assessment (O.S.U. SIGMA). Specifically we tested four cohorts of boys and girls grades k-8 (N=1,045) over nine years (1992=k-8, 1994=2-8, 1996=4-8, 1998=6-8; in '00 we again tested the original kindergarten cohort. The method of analysis was latent variable growth curve modeling. Essentially this technique uses intercepts and slopes of individual performances to establish growth trajectories. Explanatory variables (e.g., sex and sport participation) are then added to the model to provide a clearer understanding of throwing development. The principal findings include: 1) significant differences were observed between boys and girls on intercept but not on slope. In terms of throwing development boys began schooling at a higher level of performance than girls; however, growth trajectories (slope) for boys and girls were similar; and 2) when the explanatory variables of time and sex were added to the model, they contributed significantly to growth rate for each of the three cohorts. Children who participated in school and/or community-based sports (T-ball, softball or baseball), at the time of testing, demonstrated steeper growth trajectories regardless of sex, that is, they were more likely to perform a mature throw when tested.