This study describes the development and validation of the Extract the Base test (ETB), which assesses derivational morphological awareness. Scores on this test were validated for 580 monolingual students and 373 Spanish-speaking English language learners (ELLs) in third through fifth grade. As part of the validation of the internal structure, which involved using the Generalized Partial Credit Model for tests with polytomous items, items on this test were shown to provide information about students of different abilities and also discriminate amongst such heterogeneous students. As part of the validation of the test's relationship to criterion, items were shown to correlate with measures of word identification, reading comprehension, and vocabulary measures. Differences in performances for fluent English students and ELLs, students of varied home language environments, and different grade levels were noted. Additionally, the task was validated using a dichotomous scoring system to provide reliability and validity information using this alternate scoring method. (Contains 1 note, 3 tables, and 2 figures.)