The determinants of English language fluency among immigrants and the effects of fluency on earnings are examined, using a unique data set and a sample of more than 800 illegal aliens apprehended in Los Angeles, California. Analysis of the data show the importance of certain variables not previously available, speaking fluency at migration and English reading fluency. English speaking and reading fluency both increase with the time in the United States, and the increase with time is greater for those who are younger at the time of migration, who have had more schooling, and who are not Hispanic. It is shown that reading fluency is more important than speaking fluency as a determinant of earnings. It is also suggested that future surveys of immigrants should include questions on English language proficiency at arrival as well as at the time of interview. The report is divided into four sections: a review of the literature on language and earnings in the labor market for immigrants; data used for this study; a multiple regression analysis of the determinants of fluency in speaking and reading English, including longitudinal changes in speaking skills; and a regression analysis of earnings that focuses on the roles of fluency in speaking and reading English. (LB)