In this article, we evaluate the relative power of differential exposure and differential vulnerability to stressors to account for variations in psychological distress between single and married mothers. The data for this assessment are derived from a longitudinal survey of 518 single mothers and 502 married mothers living in London, Ontario, Canada. Both cross-sectional and longitudinal analyses clearly reveal that the higher levels of psychological distress experienced by single mothers compared to married mothers are almost entirely related to their greater exposure to stress and strain rather than to any group differences in vulnerability to stressful experiences. Across a number of different dimensions of social stressors, single mothers are consistently more exposed to these stressors than married mothers are. Moreover, this differential exposure persists over time. In contrast, there is no evidence that single mothers are more vulnerable or reactive to stressors than are married mothers. We discuss these findings in terms of their implications for the sociology of mental health and for primary prevention.