In this study we investigated the internal factor structure of a large-scale K--12 assessment of English language proficiency (ELP) using samples of fourth- and eighth-grade English learners (ELs) in one state. While U.S. schools are mandated to measure students' ELP in four language domains (listening, reading, speaking, and writing), some ELP standards released recently have defined ELP on the basis of integrated modalities, such as receptive language or collaborative communication. To explore whether current assessments can empirically support new conceptualizations such as these, we compared seven models based on different hypothesized structures for language proficiency. For the Grade 8 students, we find support for a hierarchical factor model, with general language underlying the four domains. A model with the four domains offered the best fit for the Grade 4 sample but fell just shy of criteria for acceptable fit. Models that incorporate more specific higher-order modalities, such as literacy or productive language, functioned less well for the given data of Grades 4 and 8 samples, suggesting the current shift in ELP definition may require shifts in how ELP assessments are built and scored.