The influence of Air Traffic Control (ATC) instruction format (grouped vs. sequential presentation) and message length on General Aviation pilot communication was investigated in a simulated flight environment using the Civil Aeromedical Institute's (CAMI's) Basic General Aviation Research Simulator (BGARS). Prior to flying the simulator each pilot was provided with familiarization training, listened to and read back ATC messages spoken in either grouped or sequential format (depending on their assigned treatment group), and completed a digit span test (a measure of short-term memory). While flying 2 missions in the simulator, 12 pilots heard recorded ATC messages that contained altitude and radio frequency information spoken in a grouped format (e.g., "descend and maintain forty-one hundred"), and 12 heard the same instructions spoken sequentially (e.g., "descend and maintain four thousand one hundred"). The amount of information in a message varied from 2 to 5 speech acts, including the aircraft identification. All pilots were instructed to read back and execute the ATC instructions. Readback errors and requests to clarify ATC messages were the primary measures of pilot communication. Readback strategies, such as whether pilots repeated instructions in the same format as issued by ATC, were also examined. We found only limited evidence that the grouped format improved pilot memory for ATC messages. In one analysis of requests for clarification, pilots who received grouped instructions produced fewer requests than did pilots who received the same instructions in sequential format, suggesting that they were less likely to misunderstand the ATC messages. Pilots who received grouped instructions were also more likely to read back the grouped instructions in sequential format, suggesting that prior experience with the sequential format influenced pilot communication in this study.