OBJECTIVE: Individual differences in stress reactivity constitute a crucially important mechanism of risk for depression. Because stress can be conceptualized as the continuous occurrence of minor daily hassles, this study focused on emotional reactivity to stress in the flow of daily life and examined to what degree individual differences in emotional reactivity could be explained by genetic and/or environmental factors. METHODS: Two hundred seventy-five female twin pairs (170 monozygotic and 105 dizygotic) participated in this experience sampling study (ESM). ESM is a validated structured diary technique assessing stressors and mood in daily life. Individual emotional stress reactivity was conceptualized as changes in negative affect in relation to appraised subjective stress in relation to daily events. Structural equation modeling was used to fit univariate models. The best fitting model was chosen based on likelihood and parsimony. RESULTS: Genetic factors (explaining 12% individual differences) and individual-specific environmental factors (explaining 88%) influenced daily life stress reactivity. CONCLUSION: The demonstration of a small genetic influence on the dynamic relationship between minor stress and affective response in the flow of daily life sheds light on the gene-environment interactions that contribute to the risk of developing stress related disorders such as depression.