Summary B dellovibrio bacteriovorus HD 100 is an obligate predator that invades and grows within the periplasm of G ram‐negative bacteria, including mcl‐polyhydroxyalkanoate ( PHA ) producers such as P seudomonas putida . We investigated the impact of prey PHA content on the predator fitness and the potential advantages for preying on a PHA producer. Using a new procedure to control P . putida KT2442 cell size we demonstrated that the number of B dellovibrio progeny depends on the prey biomass and not on the viable prey cell number or PHA content. The presence of mcl‐ PHA hydrolysed products in the culture supernatant after predation on P . putida KT42Z , a PHA producing strain lacking PhaZ depolymerase, confirmed the ability of B dellovibrio to degrade the prey's PHA . Predator motility was higher when growing on PHA accumulating prey. External addition of PHA polymer (latex suspension) to B dellovibrio preying on the PHA minus mutant P . putida KT42C1 restored predator movement, suggesting that PHA is a key prey component to sustain predator swimming speed. High velocities observed in B dellovibrio preying on the PHA producing strain were correlated to high intracellular ATP levels of the predator. These effects brought B dellovibrio fitness benefits as predation on PHA producers was more efficient than predation on non‐producing bacteria.