Temperament is an important predictor of socioemotional adjustment, such as externalizing and internalizing disorders (Kagan & Fox, 2006; Rothbart & Bates, 2006; Pérez-Edgar & Fox, 2005). However, there is not a one-to-one correspondence between temperamental predispositions and these outcomes, implying that other factors also contribute to the development of internalizing and externalizing problems. Self-regulation is believed to interact with temperament, and hence, has also been studied as an important predictor for later socio-emotional outcomes (Rothbart, 2011). Respiratory sinus arrhythmia (RSA) is a psychophysiological measure of self-regulation (Beauchaine, 2001; Porges, 2007) that has been found to moderate the relation between other types of risk (e.g., parental drinking, marital conflict, domestic violence) and socioemotional disorders (El-Sheikh, 2001; El-Sheikh et al., 2001; Katz, 2007). The primary aim of the present study was to test RSA reactivity as a moderator of the relation between temperament in infancy and externalizing and internalizing problems at kindergarten entry. Results showed that RSA moderated the relation between exuberant and fearful temperament and externalizing behaviors. However, these results were different for boys and girls. The fearful temperament predicted later internalizing problems, but no moderation was present in predicting internalizing problems. These results are discussed in light of recent evidence regarding gender differences in the role of RSA as a protective factor for risk. Finally, this study explores RSA trajectories as moderators of the relations between early temperament and socioemotional problems.