This study explored the nature of mathematics anxiety in a sample of 106 ethnically and linguistically diverse first-grade students. Although much is known about mathematics anxiety in older children and adults, little is known about when mathematics anxiety first emerges or its characteristics in young children. Results from exploratory factor analysis indicated that mathematics anxiety in first grade is a multidimensional construct encompassing negative reactions, numerical confidence, and worry. Negative reactions were related specifically to foundational mathematical concepts whereas numerical confidence was related specifically to computation skill; worry was not related to any outcome. Levels of mathematics anxiety did not differ by sex or language background. Overall, negative reactions and numerical confidence were found to be the most salient dimensions of mathematics anxiety in this sample.