  Magnetic microcalorimeters (MMCs) are cryogenic, energy-dispersive
single-particle detectors providing excellent energy resolution, intrinsically
fast signal rise time, quantum efficiency close to 100\%, large dynamic range
as well as almost ideal linear response. One of the remaining challenges to be
overcome to ultimately allow for the utilization of large-scale MMC based
detector arrays with thousands to millions of individual pixels is the
realization of a SQUID based multiplexing technique particularly tailored for
MMC readout. Within this context, we report on the first truly multiplexed
readout of an MMC based detector array using a frequency-division multiplexing
approach realized by a custom microwave SQUID multiplexer based readout system.
