Injection of fast neutrals with a component of the beam velocity along the magnetic fieldB0results in a plasma having a sheared mean velocity alongB0. If this parallel velocity shearU′and the density inhomogeneity scale lengthLnare large enough so thatU ′|Ln |/(&tgr; + 1) 1/2&ugr;i > 1then a parallel Kelvin‐Helmholtz instability is excited, whereviis the ion thermal speed and&tgr;is the electron temperature/ion temperature. Magnetic shear stabilization requires a shear lengthLscomparable to or less than|Ln |, namely,&tgr;L s /3 (&tgr; + 1) < &ugr;i /U ′ < |Ln |/(&tgr; + 1) 1/2. ForU ′|Ln |/(&tgr; + 1) 1/2&ugr;i < 1, however, this parallel Kelvin‐Helmholtz instability is stabilized and only drift wave instabilities can be excited. For&tgr; ≈ 1and(M / m) 1/2 ≈ 43, the first of these drift wave instabilities is unstable forU ′/&OHgr;i > 172| (Ln / Ls)3/2 |provided|Ln /Ls| < ai /30|Ln |, while the second is stable forU ′ /&OHgr;i < 136ai /|Ls |provided|Ls /Ln | > 600andU ′ /&OHgr;i ≪ 1, whereaiand&OHgr;iare the ion gyration radius and frequency.