A 16-kV, 4-A hydrogen beam is co-injected into hydrogen plasmas and coupled to an RF of 800 MHz, up to 150 kW. It is observed, from the measurement of parallel charge-exchange neutrals, that fast ions above 16 keV are enhanced by interaction with lower hybrid waves. This enhancement is critically reduced when the line-averaged electron density is higher than 1.6 × 1013cm−3, in the toroidal fields of both 22 and 16 kG. For the 22-kG case, the cause of the reduction may be related to the location of the turning point of the incident waves, i.e. the turning point, in higher-density regions, is located outside the region where the fast ions produced by NBI are relatively abundant. For the 16-kG case, however, this interpretation is only valid when a portion of the incident wave energy is transferred to waves of shorter wavelength. The ion temperature increase for simultaneous heating agrees with the sum of those in the individual heatings even when the beam-RF interaction is rather strong.