Negative emotionality is linked to unfavorable life outcomes, but studies have yet to examine negative emotionality of parents and children as predictors of children's problem behaviors and negative emotion word use in everyday life. This study used a novel naturalistic recording device called the Electronically Activated Recorder to investigate the separate and interactive influences of parent and child negative emotionality on daily child behaviors in a sample of 35 preschool-aged children over two time points separated by 1 year. Fathers' negative emotionality predicted children's whining at Time 1; mothers' negative emotionality predicted children's negative emotion word use at Time 1 and increases in children's arguing/fighting from Time 1 to Time 2. Parents' ratings of child negative emotionality also were associated with increases in children's arguing/fighting from Time 1 to Time 2, and child negative emotionality moderated the association between mothers' negative emotionality and children's arguing/fighting. Further, children with mothers high in negative emotionality displayed higher levels of problem behaviors when their mothers self-reported low levels of positive emotional expressiveness and/or high levels of negative emotional expressiveness. These findings offer preliminary evidence linking parent and child negative emotionality to everyday child behaviors and suggest that emotional expressiveness may play a key role in moderating the links between maternal negative emotionality and child behavioral problems.