Objectives The purpose of this study was to examine the relationship between girls’ sexual maturation (age of menarche) and physical activity and sedentary behaviors. Methods Data were collected from a national representative sample of girls in 2010 (pre‐menarcheal girls n = 583, post‐menarcheal girls n = 741). Physical activity (times/week and hours/week) and screen‐based sedentary time (minutes/day) including television/video/DVD watching, playing videogames, and computer use were self‐reported. Results Pre‐menarcheal girls engaged significantly more times in physical activity in the last 7 days than post‐menarcheal girls (3.5 ± 1.9 times/week vs. 3.0 ± 1.7 times/week, P < 0.001). There was no significant difference between pre‐menarcheal and post‐menarcheal girls in time (hours/week) spent in physical activity. Post‐menarcheal girls spent significantly more minutes per day than pre‐menarcheal girls watching TV, playing videogames, and using computers on weekdays (TV: 165.2 ± 105.8 vs. 136.0 ± 106.3, P < 0.001; videogames 72.0 ± 84.8 vs. 60.3 ± 78.9, P = 0.015; computer: 123.3 ± 103.9 vs. 82.8 ± 95.8, P < 0.001) and on weekends (TV: 249.0 ± 116.2 vs. 209.3 ± 124.8, P < 0.001; videogames: 123.0 ± 114.0 vs. 104.7 ± 103.5, P = 0.020; computer: 177.0 ± 122.2 vs. 119.7 ± 112.7, P < 0.001). After adjusting analyses for age, BMI, and socioeconomic status, differences were still significant for physical activity and for computer use. Conclusion Specific interventions should be designed for girls to increase their physical activity participation and decrease time spent on the computer, for post‐menarcheal girls in particular. Am. J. Hum. Biol. 28:471–475, 2016. © 2015 Wiley Periodicals, Inc.