This study compares 2 measures of reading comprehension: (a) the Woodcock-Johnson Passage Comprehension test, a standard in reading research, and (b) the Diagnostic Assessment of Reading Comprehension (DARC), an innovative measure. Data from 192 Grade 3 Spanish-speaking English language learners (ELLs) were used to fit a series of latent variable analyses designed to explicitly test the discriminant validity and differential determinants of the 2 measures. Findings indicated that the 2 measures are related (r = 0.61) but distinct, and influenced by different factors. The DARC is less strongly related to word-level skills and more strongly related to measures of narrative language production and memory. Both tests are equally influenced by measures of nonverbal reasoning. These differential patterns of relations, which cannot be explained on the basis of differential reliabilities, reflect true differences in the processing demands of the tests for 3rd-grade ELLs.