We show that the ownership structure of corporate bonds, particularly the portfolio constraints and trading behavior of bondholders, shape how monetary policy surprises transmit to CDS. Using high-frequency policy shocks, we find that CDS spreads react more sharply when a firm's bonds are held by investors whose portfolios are concentrated in low-rated securities (+11.2 basis points per 1 p.p. tightening) or long-duration instruments (+9.9 basis points), consistent with binding portfolio constraints that force rebalancing. Bonds with higher mutual fund ownership initially appear substantially more sensitive (+79 basis points), but this effect diminishes when controlling for the trading intensity of the bondholder base. The deeper mechanism is behavioral rather than institutional: bonds held by high-turnover investors exhibit larger CDS responses because these investors rebalance aggressively when monetary conditions shift, generating concentrated order flow that spills from cash bond markets into CDS pricing through hedging demand and basis arbitrage. Our findings imply that monitoring who holds bonds and how actively they trade is at least as important as monitoring what type of institution they are for understanding how monetary policy propagates through credit markets.