This article analyses if and to what extent labour market segmentation and labour market mobility influence trade union density. Some industries and sectors have stable employment domains and employees stay to a high degree within the industry even if they change jobs. Other industries and sectors have more unstable employments domains and employees to a higher degree shift to employment in other industries and sectors when they move to another job. In this article, it is analysed how differences in segmentation and employee mobility out of an industry influence union density. The analysis is based on a statistical analysis of registry data from Denmark and contains almost 2 million employees employed in 111 different industries (NACE-coded). The analysis shows that trade union density especially in the private sector industries is significantly influenced by level of segmentation and level of mobility.