Photoexcited defect clusters in the bulk of KH2PO4 crystals are investigated using a microscopic fluorescence imaging system with 1 μm spatial resolution. The observed defect cluster concentration is approximately 104–106 per mm3 depending on the crystal growth method and sector of the crystal. The intensity of the emission clusters varies widely within the image field while a nearly uniformly distributed background is present. Spectroscopic measurements provided information on the emission characteristics of the observed defect population. © 1999 American Institute of Physics.