Diatoms are unicellular photosynthetic algae with enormous diversity of patterns in their silica structures at the nano- to micronscale. In this study, we present results, which support the hypothesis that silica nanoparticles are released into the diatom culture medium. Formation of an opalescent film by the self-assembly of silica nanoparticles produced in the growth medium of diatoms. This film was formed on the filter paper from the culture medium of a Coscinodiscus sp. culture. A number of diatoms with partially opened valves were observed on the film surface under light microscopy and SEM, which indicates that cell contents inside of diatoms had been released into the culture solution. AFM images of produced an opalescent films show ordered arrays of silica nanoparticles with different diameters depending on the colors observed by light microscopy. The film forming silica nanoparticles are either released by the diatoms during reproduction or after cell death This approach provides an environmentally friendly means for fabricating silica nanoparticles, decorative coatings and novel optical materials.