Low and high beta frequency rhythms were observed in motor cortex, but their respective sources and behavioral correlates remain unknown. We studied local field potentials during pre-cued reaching behavior in macaques. They contained a low beta band (<20Hz) dominant in primary motor cortex and a high beta band (>20Hz) dominant in dorsal premotor cortex. Low beta correlated positively with behavioral reaction time, from visual cue onset, and negatively with uninstructed hand postural micro-movements throughout the trial. High beta reflected temporal task prediction, with selective modulations before and during cues that were enhanced in moments of increased focal attention, when the gaze was on the work area. This double-dissociation in cortical sources and behavioral correlates of motor cortical low and high beta, with respect to both task-instructed and spontaneous behavior, reconciles the largely disparate roles proposed for the beta rhythm, by suggesting band-specific roles in both movement control and spatio-temporal attention.