Previous research has found that genetic and nonshared environmental factors influence marital quality (Spotts et al., 2004). The current study explored personality as a source for these genetic and environmental individual differences. A sample of 752 Swedish twin women and their spouses were used. Genetic and environmental influences were found for self-report measures of marital quality, but only environmental factors contributed to the variance of observational measures of marital quality. Wives' personality characteristics accounted for genetic and nonshared environmental variance in the wives' own marital satisfaction, their husbands' marital satisfaction, and the agreement between the spouses on the quality of their marriage. Genetic influences on the correlation between wives' genetically influenced personality characteristics and their husbands' marital satisfaction indicate a gene–environment correlation. Contrary to expectations, husbands' personality did not explain large portions of wives' marital satisfaction beyond that explained by wives' personality. This study emphasizes the importance of spousal personality to the well-being of marriages, and results are discussed within the context of three different theories regarding associations between personality and marital quality. Copyright © 2005 John Wiley & Sons, Ltd.