An adjustable gas injector (10) for injecting a gas into a liquid at a controlled rate. The injector (10) includes a housing (12) and a gas flow adjustment device (56). The housing (12) includes primarily a liquid inlet (14), a gas inlet (18), a gas/liquid mixture outlet (32), and a mounting device (44). A mixing chamber (36) is defined between the liquid inlet (14) and the mixture outlet (32). A liquid stream is directed through the chamber (36) where it is mixed with a gas. In the direction of flow, the mixing chamber (36) defines a first region (38) in which the diameter decreases, a throat (40), and a second region (42) in which the diameter increases. The gas inlet (18) opens into the chamber (36) proximate the throat (40). The gas inlet (18) defines first and second channels (26,30). The housing (12) defines an opening (52) collinear with at least a portion of the first channel (26) of the gas inlet (18). At least a portion of the opening (52) defines a threaded receptor (54). The gas flow adjustment device (56) includes a spring-biased-ball (58) seated within the opening (52) against the distal end (28) of the first gas inlet channel (26). A threaded insert (62) is engaged in the threaded receptor (54) to control the pressure applied to the spring (60), and thus control the flow rate of the gas into the chamber (36).