Many human behaviors (e.g., cognitive, moral, and psychosocial) follow predictable developmental patterns or stages. The study reported examined the interpersonal development of 9- through 13-year-old children. A total of 282 children were administered the Fundamental Interpersonal Relations Orientation - Behavior Children (FIRO-BC) test to identify and describe a stage-like progression in interpersonal behavior development. The data presented here failed to evidence such a progression. Rather, interpersonal behavior was largely unrelated to age until children reached 13, whereupon their profiles were significantly different from those of younger children. Similarly, gender-related differences appeared for the first time in 11-year-olds and increased dramatically by age 13. These two facts suggest that interpersonal behavior may be more clearly related to puberty than has been previously supposed. (Author/RH)