The remarkably well-preserved fossils in the Florissant Formation (Eocene) at Florissant, Colorado occur in a lacustrine shale consisting of numerous couplets of millimeter-thin, alternating laminae of diatomite and volcanic debris of ash and smectite clay. The results of this study indicate the importance of microbial processes in fossilization. The plant and insect fossils are found within the diatomite layers and are covered with a mucus film produced by diatoms. The laminated shale sequence represents cyclic sedimentation during which ash-clay sedimentation alternated with intervals of abundant diatom growth (blooming?). During each proliferation of diatoms, a mat of diatom mucus formed. Additional mucus came from secretions of bacteria lying on the diatom mat. Bodies of organisms blown or washed into the lake became entangled in the sticky mucus mat that later settled to the lake bottom. The mucus retarded decomposition of the plants and insect bodies while sinking through the water column and during the interval before burial on the lake floor. The fossil-containing diatomite layer was then buried under an ash-clay layer. The organisms escaped decomposition and were fossilized mainly because of the protective mucus covering.