Using data on the near-universe of US online job vacancies collected by Burning Glass Technologies in 2016, we calculate labor market concentration using the Herfindahl-Hirschman index (HHI) for each commuting zone by 6-digit SOC occupation. The average market has an HHI of 4,378, or the equivalent of 2.3 recruiting employers. 60% of labor markets are highly concentrated (above 2500 HHI). Highly concentrated markets account for 16% of employment. Labor market concentration is negatively correlated with wages, and there is no relationship between measured concentration and an occupation's skill level. These indicators suggest that employer concentration is a meaningful measure of employer power in labor markets, that there is a high degree of employer power in labor markets, and also that it varies widely across occupations and geography.