The structure of temperament proposed by the New York Longitudinal Study (NYLS) of Thomas and collaborators (1963, 1968) was compared to Buss and Plomin's (1975) EASI temperament theory. An objective inventory was developed for the NYLS temperaments. Mothers rated 182 children on both NYLS and EASI temperaments. Only two of the nine NYLS dimensions, attention span-persistence and distractibility, were supported by factor analysis. The four EASI dimensions (emotionality, activity, sociability, and impulsivity) were replicated. A similar sociability factor emerged from both the NYLS and EASI systems, but other temperaments were unique. The NYLS and EASI systems were merged to form the Colorado Childhood Temperament Inventory, a parental rating instrument for children 1-6 years of age.