Research was conducted to determine if alterations in the acoustical characteristics of voice occur over periods of sustained wakefulness. Twelve male United States Air Force B-1B aircrewmen participated in the study. The participants served in teams of four and performed three 36-hour experimental periods (missions) in a high-fidelity simulator. The missions were interspersed with 36-hour rest breaks. Data were lost from two members of the third team due to a communication malfunction. Speech, cognitive, and subjective fatigue data were collected approximately every three hours for 11 trials per mission. Fundamental frequency and speech duration were both found to vary significantly over trials (fundamental frequency F(10,90)=2.63, p=0076, speech duration F(10,90)=2.5, p=.0106). Speech duration results also showed a significant main effect of mission (F(2,18)=6.91, p=.0082). The speech data follow the same trend as the data from the cognitive tests and subjective measures. A strong diurnal pattern is reflected in nearly all of the dependent measures. Overall, the results support the proposition that voice may be a reliable and valid indicator of a speaker's fatigue state.