A high-density, very low power (<<500 (mu) W/channel) readout system for the Hamamatsu R5900-00-M64 and similar multianode photomultiplier tubes (MAPMTs) is under development at NOVA R&D. It is intended for space-flight instruments involving a large number of channels, and provides for efficient readout of such a system at low to moderate event rates in the presence of a sparse hit pattern. Data is buffered within the readout ASIC while prompt summary information is made available to the instrument trigger system, which can then cause the data to be read from the buffer only when necessary. A prototype of the analog front- end has been designed and fabricated, and a prototype of the complementary digital functions has been designed and implemented in a field-programmable gate array (FPGA). We report here on these designs and on test results of the prototype readout system with the Hamamatsu MAPMT. Performance parameters such as gain, noise, and trigger threshold have been studied, and pulse height spectra with a single- photoelectron source and with a Sr90 source and scintillating fiber have been obtained.