How closely do observed, phenotypic personality structures correspond with underlying, genetic, and environmental personality structures? This question was addressed using data from 2,490 pairs of twins from the Minnesota Twin Registry, who completed A. Tellegen's (1982) Multidimensional Personality Questionnaire (MPQ). Phenotypic, additive genetic, and nonshared environmental correlations were computed among the MPQ's eleven primary scales. Principal component analyses of all three matrices revealed three broad, higher-order components that were readily interpreted as negative emotionality, positive emotionality, and constraint. The components from all three matrices were highly comparable, suggesting that the observed, phenotypic structure of personality corresponds closely with the underlying, etiological structure of personality.