A new type of inorganic nanoparticle-organic hybrid crystalline material consisting of gold nanoparticles (Au NPs) and p-hydroxyacetanilide (pHA) is reported. The composite crystals were on the order of several millimeters in dimensions. They could be grown from a solution of Au NPs and pHA at 35 degrees C. The optical properties of the crystals not only reflected the presence of Au NPs but also their degree of association inside the crystals. Single crystal X-ray diffraction data indicated that the crystal motifs were those of pHA. Transmission electron microscopic images indicated Au NPs being dispersed randomly in the crystal with increase in their density when crystallized in the presence of low concentration of pHA. FTIR measurements indicated attachment of -NH group to the NPs. Optical microscopic investigation revealed the presence of Au NP crystals, the color of which represented their density, being red at low concentration of NPs and purple at their high concentration.