One explanation for the relatively low scholastic achievement of deaf students is the character of communication in the classroom. Unlike aural communication methods, line-of-sight methods share the limitation that the receiver of the message must look at the sender. To assess the magnitude of this constraint, we measured the amount of time signers were looked at by potential receivers in typical secondary school classes for the deaf. Videotaped segments indicated that on average the messages sent by teachers and students were seen less than half the time. Students frequently engaged in collateral conversations. The constraints of line-of-sight communication are profound and should be addressed by teaching techniques, classroom layout, and possibly, the use of computer-communication technology.