Children in the United States and in other English-speaking countries often learn a good deal about letters before they begin formal reading instruction. We suggest that one important and previously unrecognized type of knowledge about letters is knowledge of the phonological structure of the letters' names. In two experiments, preschoolers with a mean age of 4;8 judged whether various syllables were letters. The children made significantly more false positive responses to syllables such as /fi/, which have a phonological structure shared by a number of letters, than to syllables such as /fa/ and /if/, which sound less like real letters. This was true even for children who could recite the alphabet without error. Learning the alphabet, we conclude, forms the basis for generalizations about the structure of letter names.