A drug delivery device (1), comprising a delivery unit (2) in the form of a disposable single-use part and a base unit (4) in the form of a reusable part for coupling to the delivery unit (2), the delivery unit comprising a liquid flow system for pumping liquid from a drug vial or cartridge (3) received in the drug delivery device to an outlet of the device for transcutaneous administration of a liquid drug, the base unit including at least a power source and an electronic control system, the delivery unit and base units comprising a releasable locking mechanism including a locking shoulder (30) on the delivery unit that engages a complementary locking shoulder (34) on the base unit. The locking mechanism comprising an unlocking actuator that may be actuated to release the locking shoulder (30) from the complementary locking shoulder (34). The device further comprises a coupling member (8b) on the delivery unit that engages a complementary coupling member (8a) on the base unit, at least one of the coupling member and complementary coupling member comprising at least one elastic arm (10) with a protuberant portion (11) at a free end thereof, the protuberant portion (11) having a rearwardly inclined taper (13) that engages a complementary taper on a complementary protuberant latch portion (12) of the other of said coupling member and complementary coupling member, configured to provide a pulling force (F) between the base unit and the delivery unit to draw them together in a tight fit without play.
