We provide evidence of excess comovement in the credit default swap (CDS) market following inclusions to and exclusions from investment grade and high yield CDX indices during the 2003–2016 period. We find that when a name joins an index, its return tends to covary more with the returns of that index and conversely when it is excluded from an index, its return tends to covary less with it. We use univariate regressions and a difference-in-difference approach to show that the CDS market is impacted by indexation. This excess comovement indicates a departure from fundamental-based pricing and provides support in favour of style investing.