An objective of the present study was to conduct a comprehensive assessment of pre-kindergartenchildren’s informal mathematical knowledge, an assessment that examined not just numerical knowledge but also knowledge of arithmetic, space and geometry, measurement, patterns, and logical relations.Our assessment revealed that more mathematical knowledge had developed in young children frommiddle-income families than in children from low-income families. This SES-related gap in mathematical knowledge was present and was conceptually broad at both the beginning and the end of thepre-kindergarten year. A preliminary estimate of the developmental extent of the gap was made by comparing the CMA scores of two groups of children who had not received a mathematics intervention. Thescores of middle-income children at age 4 years, 3 months were significantly higher than the scores oflow-income children at age 4 years, 10 months, suggesting a gap of at least 7 months prior to 5 years of age.