Labor markets in advanced economies have undergone substantial change in recentdecades due to globalization, technological improvements, and organizational changes. Due tothese developments, oral and written language skills have become increasingly important evenin less skilled jobs. Immigrants – who often have limited skills in the host country languageupon arrival – are likely to be particularly affected by the increase in language requirements.Despite this increase in literacy requirements, little is known about how immigrants’ languageproficiency is rewarded in the labor market. However, estimating the causal effect ofimmigrants’ language skills on hiring is challenging due to potential biases caused by omittedvariables, reverse causality, and measurement error.To address identification problems, we conduct a large-scale field experiment, where wesend thousands of fictitious resumes to employers with a job opening. With the help of aprofessional linguist, we manipulate the cover letters by introducing common second-languagefeatures, which makes the resumes reflect variation in the language skills of real-worldmigrants. Our findings show that better language proficiency in the cover letter has a strongpositive effect on the callback rate for a job interview: moving from the lowest level of languageproficiency to a level similar to natives almost doubles the callback rate. Consistent with therecent development that language proficiency is also important for many low- and mediumskilledjobs, the effect of better language skills does not vary across the vastly different typesof occupations we study. Finally, the results from employer surveys suggest that it is improvedlanguage skills per se that is the dominant explanation behind the language proficiency effect,rather than language skills acting as a proxy for other unobserved abilities or characteristics.