The relation and agreement of tri-axial accelerometry and heart rate telemetry in measuring moderate to vigorous physical activity were examined in association to behavioral observation during 1st- and 2nd-grade physical education. In Study 1, physical activity measures of heart rate and behavioral observation were collected on 346 participants ages 6-9 years (M = 7.27, SD = 0.58). In Study 2, participants (N = 125) were 6-9 (M = 6.98, SD = 0.72) years old and had their physical activity measured via tri-axial accelerometry and behavioral observation. Tri-axial accelerometry and observation (r = 0.77 to 0.79, p less than or equal to 0.01) demonstrated a stronger relation than heart rate and observation (r = 0.42 to 0.49, p less than or equal to 0.01). Bland-Altman plots (Bland & Altman, 1986) revealed a marginally better agreement between tri-axial accelerometry and observation than between heart rate and observation, but both accelerometry and heart rate telemetry significantly overestimated physical activity time. In conclusion, neither accelerometry nor heart rate, following the instrumentation data collection protocol in this study, would be an acceptable alternative for behavioral observation. (Contains 2 tables and 2 figures.)