We propose a generalization of standard differential phase-shift keying (DPSK) to multiple-antenna wireless communication. DPSK is commonly used for single-antenna mobile wireless communication on an unknown rapidly-fading channel. Using multiple transmitter antennas at the basestation, we propose a differential modulation scheme that promises large performance and diversity gains over standard DPSK when communicating with a mobile. The mobile receiver does not need to know the channel and can operate with only a single antenna.