The relationship between transient mood phenomena and enduring personality characteristics was studied. Eighty-one women completed the NEO Personality Inventory (Costa & McCrae, 1985) and provided daily unidimensional mood ratings for 20 days. Average mood was negatively related to Neuroticism (N) and positively related to Extraversion (E). More fine-grained analyses revealed that Positive Emotions, a facet of E, was a better predictor of average mood than was the overall E factor score. The greater a facet's loading on N or E, the better it predicted average mood. Within-day mood variability (mood fluctuation) was positively related to Openness to Experience (O), particularly to the facet, Fantasy. Across-day variability (mood swing) was positively related to both E and O. No relationship was found between any of the mood variables and Agreeableness (A) or Conscientiousness (C). The 3 mood variables were virtually independent of one another, but each showed moderately high temporal stability across the 4-week period.