The vocabulary used in textbooks and standardized tests for English as a second language (ESL) is examined to determine how much variation exists. Word lists were compared for six difficulty levels in five frequently-used ESL textbook series (a total of 30 books). Three series of standardized proficiency/placement tests at different levels were studied for commonalities. Noteworthy findings for the textbooks include these: almost half of the 7,581 vocabulary words studied were taught in only one series at any level; only about 1 percent were taught in all series at the same level, and they were generally very basic words taught at the first level; and less than 10 percent were taught in all 5 series but at different and widely varying levels. Analysis of the tests found that many of the vocabulary items tested were not found in any of the textbooks at any level. The study concludes from these results that the lack of uniformity in vocabulary taught to ESL students may be seriously affecting the students' preparation for language use and later study. It recommends that the profession establish a core vocabulary that uniformly and systematically introduces given words at specified proficiency levels in all texts, to be supplemented with additional words of choice. (MSE)