This article discusses some of the challenges that researchers working in the fields of bilingualism and second-language acquisition (SLA) and in the field of language testing face in developing comparable and culturally and cognitively appropriate data collection and language assessment tools for bilingual children from rural minority-language communities with low levels of literacy. These challenges include defining language proficiency in a bilingual continuum, avoiding cultural biases, and attaining cross-linguistic validity. Using evidence from oral proficiency and aural comprehension tasks developed for bilingualism and SLA research and for the assessment of language proficiency in Quechua-Spanish and Aymara-Spanish bilingual students in the public schools of Peru, I illustrate recent attempts to develop appropriate tests to probe into bilingual students' linguistic competence. (Contains 8 footnotes, 2 tables and 5 figures.)