This study examines the acquisition of Japanese numeral classifiers in Japanese preschool children, ages 3 to 6, with a primary emphasis on developing comprehension ability. Numeral classifiers, widely distributed in languages of East and Southeast Asia and the New World, are a group of morphemes that usually occur adjacent to quantity expressions. The selection of numeral classifiers is determined by the inherent semantic properties of the noun whose quantity is being specified, suggesting that developing patterns of comprehension should be linked to underlying patterns of semantic and conceptual development. Previous research claims that children acquire certain distributional patterns very early but that the acquisition of the semantic system is a very slow process. We argue instead that different techniques and stimulus contrast sets reveal a much greater sensitivity to semantic relations in young children than was previously considered possible. Reasons for the apparent slowness in classifier acquisition are also discussed.