Let Sleeping Corpses Lie (1974)

A Spanish/Italian production shot in England, this film follows the travails of a hippie (Ray Lovelock) and his chance traveling companion (Christine Galbo) who become suspects in a series of murders committed by the living dead. They are relentlessly pursued by the zombies and by The Man, primarily a relentless police inspector played by Arthur Kennedy. The zombies are a product of a new agricultural device to kill insects, a nice idea which may have been produced by the burgeoning environmental consciousness of the time.

Director Jorge Grau borrows somewhat from Night of the Living Dead, but also creates realistically creepy zombies and novel scares, with harrowing sequences in a mortuary basement and a hospital.  Darkly atmospheric and full of nicely-done grue, it's well worth seeing.  

Note:  This is a zombie film of many names.  Besides Let Sleeping Corpses Lie, it's also been titled Don't Open the Window and The Living Dead at Manchester Morgue, among others.