The American Association on Intellectual and Developmental Disabilities has promulgated various models of adaptive behavior, including its 1992 model that highlighted 10 adaptive skills and its 2002 model that highlighted three conceptual domains. The Adaptive Behavior Assessment System-II (ABAS-II) was designed to be consistent with these models. Previous studies using ABAS-II data have provided stronger support for a one-factor model of adaptive behavior and weaker support for a three-factor model. This study extends this review by examining the factor structure of the construct of adaptive behavior as measured by the ABAS-II Teacher Form, ages 5 to 21, for males and females from two age groups by addressing the following four issues for males and females: whether (a) the skill areas in the ABAS-II Teacher Form, ages 5 to 21, display the same pattern of factor loadings, (b) the skill areas display the same factor loadings, (c) the intercepts of the observed skill areas are equal, and (d) the skill areas measure the corresponding factors with the same accuracy. The finding of invariance across males and females helps ensure the scale's factor structure is similar for the two groups. The ABAS-II Teacher Form was found to display a similar one-factor structure for males and females ages 5 to 12, 13 to 21, and 5 to 21. (Contains 4 tables.)