Abstract : The acoustic environment is critical in Air Traffic Control (ATC), as operators exchange information with planes and must also be aware of the occurrence of auditory alarms. In such situations, observing inattentional deafness is likely. In this study, we aimed to identify the physiological indicators of inattentional deafness through the analysis of the P300 evoked potential, known to be an indicator of attention allocation, an important step to a stimulus reaching consciousness. Based on the assumption that the high mental load generated by an ATC task may reduce the alarm detection rate, we wished to test whether this effect would be reflected in the alarm-evoked P300 amplitude. Participants had to perform simulated ATC tasks within the LABY microworld while electroencephalographic (EEG) activity was recorded. Simultaneously to the LABY tasks, participants were asked to respond to target tones (the “alarm”) and to ignore standard tones. Behavioral results showed that 4.6% of alarms were not reported. For these alarm detection failures, the EEG analysis showed a diminution of the P300 amplitude in comparison to a control condition in which participants only focused on the tones. These results suggest that the P300 amplitude seems to be a valid physiological indicator of vulnerability to inattentional deafness in complex environments. Relevant applications include the prevention of alarm omission and the assessment of warning designs.