  We report measurements characterizing the performance of a kinetic inductance
detector array designed for a wavelength of 25 microns and very low optical
background level suitable for applications such as a far-infrared instrument on
a cryogenically cooled space telescope. In a pulse counting mode of operation
at low optical flux, the detectors can resolve individual 25-micron photons. In
an integrating mode, the detectors remain photon noise limited over more than
six orders of magnitude in absorbed power from 70 zW to 200 fW, with a limiting
NEP of 4.6 x 10^-20 W/rtHz at 1 Hz. In addition, the detectors are highly
stable with flat power spectra under optical load down to 1 mHz. Operational
parameters of the detector are determined including the efficiency of
conversion of the incident optical power into quasiparticles in the aluminum
absorbing element and the quasiparticle self-recombination constant.
