This study investigated the mean-level and individual-level trends as well as the genetic and environmental sources of rank-order continuity and change in temperament traits (Briskness, Perseveration, Sensory Sensitivity, Emotional Reactivity, Endurance, and Activity). We analyzed self-reports and peer ratings from 2 adult age groups of twins, aged 15–30 (n = 348; 113 monozygotic and 61 dizygotic twin pairs) and 31–67 (n = 352; 110 monozygotic and 66 dizygotic twin pairs), captured at 2 different points of time 13 years apart. Temperament traits were fairly stable accompanying high levels of heritability. However, they also undergo mean-level, individual-level, and rank-order changes. In young adulthood, rank-order change was due to both genetic and environmental factors, whereas in older adult years, rank-order change was predominantly attributable to environmental effects. Phenotypic rank-order continuity was larger for older adults due to higher genetic as well as environmental continuity. For specific temperament traits (e.g., Activity), environmental variance proportionally increased across adulthood compared with genetic variance resulting in decreasing levels of heritability, whereas the reverse was found for other traits (e.g., Endurance). Specifics regarding the traits studied were discussed. The results indicate a complex interplay between genetic and environmental sources resulting in continuity and change of temperament across adulthood.