Efforts are under way to develop a wearable haptic display that can impart directional information on a user's back for situation awareness. To date, two studies have been conducted aboard the NASA KC-135A reduced gravity aircraft to investigate the perception of tactile information in altered-gravity environments. This paper reports our results on perceived loudness of vibrotactile stimulation under different gravity conditions. Subjects compared seven fixed-frequency varying-amplitude vibrations in 1.8-g to a reference vibration delivered in zero-g using the method of constant stimuli. Our results show that the points of subjective equality (PSE) measured in 1.8-g are essentially the same as the intensity of the reference signal delivered in zero-g. The difference between PSE and the reference is less than the difference threshold (DL) measured in 1.8-g. We also found that the displacements (measured with an accelerometer) produced by our tactors in one-g and zero-g conditions are the same using identical driving waveforms. Our data suggest that the perceived loudness of vibrotactile stimuli remains the same in altered-gravity environments. However, a user's ability to interpret vibrotactile signals in zero-g environment may be hampered by increased cognitive load due to the need to continuously monitor the position and movement of one's body.