The purpose of this study is to explore changes in career mobility in the US labour market during the late 1990s and early 2000s, a period in which career boundaries weakened and workers’ employment options became increasingly flexible. Using multiple panel data of a nationally representative sample of US employees between 1990 and 2003, the pattern of workers’ short-term movement across various types of boundaries in the labour market is analysed, as well as change over time and by skill group. The result shows that although the probability of switching firms increased for all workers, the career trajectories of lower-skilled groups showed increasingly opposite trends from those of higher-skilled groups. In particular, occupational immobility was reduced significantly for workers in lower-skilled occupations, yet their changes of occupation occurred mainly within their origin class, resulting in strengthening of class boundaries. Implications of this finding are discussed in light of recent debate on class stratification.