The use of glottal stops in continuous speech in American English has not been well studied, in spite of the common belief that they may serve as boundary markers. This paper reports the result from measurements of five speakers' readings of four stories, each arranged in both list and story form. Data were analyzed according to the difference in speakers, materials, versions, phonological situations (phoneme sequence and stress situations), grammatical conditions, and frequency of occurrence of words. Results show that higher-level factors such as speaker's reading style and difficulty of material have a strong influence than phonological and grammatical factors.