Physical education programs are believed to contribute to the amount of physical activity experienced by youth. Knowledge on how much physical activity individuals with disabilities are getting during instruction is important for program providers to determine the potential impact of curricular offerings on health related fitness. Activity monitors have been used to measure daily physical activity of wearers; however, short term use of these monitors for individuals with visual impairments has not been studied. The purpose of this study was to determine estimates of validity and reliability for RT3TM monitors when worn by 19 adolescents with visual impairments during physical education. These activity monitors yielded vector magnitude (VM) values which are the result of motion sensed in three planes. These values were then compared to observational data using the Children’s Physical Activity Form (CPAF) (O’Hara, Baranowski, Simmons-Morton, Wilson, & Parcel, 1989). Estimates of reliability for the RT3TM monitors were calculated by having individuals with visual impairments wear two monitors during data collection and then determining relationships between VM values for each monitor (r = .90). Values for VM and CPAF were correlated for each minute studied resulting in a very strong relationship between RT3TM values and CPAF scores (rs = .88, p < .001, n = 19). Further, over the 703 minutes studied, RT3TM scores were related to CPAF values for the respective minute intervals (rs = .78, p < .001). Finally, activity intensities from the observational system (based on combinations of categories: static no movement, stationary limb movement, slow trunk movement, and rapid trunk movement) were rank ordered to determine if mean VM per minute by participants were consistent throughout CPAF intensity categories. In this analysis, mean VM values for the respective activity intensities were consistent with the CPAF activity points (60 CPAF act pts = 115 VM, n = 165; 90 CPAF act pts = 491 VM, n = 43; 120 CPAF act pts = 596 VM, n = 272; 140 CPAF act pts = 890 VM, n = 9; 150 CPAF act pts = 1296 VM, n = 95; 160 CPAF act pts = 1378 VM, n = 66; 180 CPAF act pts = 1942 VM, n = 22; 210 CPAF act pts = 2211 VM, n = 29, and 240 CPAF act pts = 2256, n = 2). Encouraging validity and reliability estimates indicate that the RT3TM is a useful tool for measuring short term physical activity levels in adolescents with visual impairments.Keyword(s): adapted physical activity, physical activity, research