Children's natural learning of word meanings while reading was investigated in a study involving 447 American and Chinese children in third and fifth grades. The children read one of two cross-translated stories and then completed a test on the difficult words in both the story they read and the one they did not read. The results showed significant incidental learning of word meanings in both grades in both countries. In each country, incidental word learning appeared on both easy and difficult test questions and among children of all levels of ability. For children from both cultures, the strength of contextual support in the stories and the conceptual difficulty of the words affected learning. The morphological transparency of words influenced word learning among Chinese fifth graders, but not among American children in either grade. Considering the many differences between China and the United States in language and culture, the results imply that incidental acquisition of word meanings while reading is a universal in written language development.