We investigated the roles of sex and respiratory sinus arrhythmia (RSA), an index of autonomic parasympathetic nervous system activity, as predictors of codeveloping externalizing and internalizing symptoms in middle childhood. We expected that sex, baseline RSA (RSA-B), and RSA reactivity (RSA-R) to two types of tasks would interact to differentiate co-occurring trajectories of symptoms. We tested these hypotheses by combining longitudinal data from two independent samples (n = 390; 210 girls, 180 boys) with repeated measures at ages 8, 9, 10, and 11. RSA-R was measured in response to a socially stressful and frustrating stressor. Indicators of growth in externalizing and internalizing symptoms were derived from multiple domain growth models and used in person-centered growth mixture analyses. Three groups of externalizing and internalizing trajectories were found. Profile membership was predicted by several two-way interactions among sex, RSA-B, or RSA-R but was not predicted by three-way interactions. Children with low RSA-B and strong RSA withdrawal, girls with low RSA-B, and girls with strong RSA withdrawal were more likely to be on a developmental trajectory of low externalizing symptoms and moderately elevated internalizing symptoms. Membership in the high externalizing and high internalizing trajectory was predicted by weak RSA withdrawal for boys and strong RSA withdrawal for girls. The type of stressor task also played a role in predicting probability of profile membership. Results are discussed in the context of developmental psychobiology and implications for the codevelopment of psychopathology symptoms in childhood.