Intersexual differences in juvenile behavior in invertebrates are poorly understood despite the recognition that they may be widespread. We present a study designed to explore sex differences in behavior in early instar long-bodied cellar spiders Pholcus phalangioides (Fuesslin 1775). Our findings reveal that sex differences in activity and feeding are present early in P. phalangioides, which may have important implications for studies of behavior that involve juveniles. Further investigation of the factors that underlie the early emergence of sex differences in behavior is needed.