This article presents evidence that turnover is negatively selective on a worker's job performance. At establishments with about 17 employees, workers who are 1 SD (21%) less productive than average during the first few months on the job are 11 percentage points more likely to be laid off or fired and 7 percentage points more likely to quit during the succeeding year. At large nonunion establishments and in small labor markets, productivity has large effects on involuntary separations but almost no effect on quits. Productivity appears to be positively related to layoffs and quits at unionized establishments.