ABSTRACT: Studies of the non-marine diatom assemblages from Eocene deposits in North America extend the fossil record of a number of extant freshwater diatom lineages. These and other accounts show that the earliest documented appearances of non-marine diatoms consisted of 7 genera from late Cretaceous deposits, followed by 36 more from the Eocene, and 4 additional genera from Oligocene to early Miocene localities. A total of 47 diatom genera, 45 of which are extant, therefore, are known from pre-Neogene (>23.8 Ma) non-marine deposits. The chronological extension of these first-occurrences of genera has implications for terrestrial diatom biostratigraphy, paleoclimate, and diatom phylogenetic studies. An overview of the pre-Neogene non-marine diatom biochronology from published records and original work provides the geologic and geographic context for the more current findings. Two geologic range charts synthesize the data from the pre-Neogene non-marine (fresh and brackish water) diatom floras from the 9 published localities, supplemented by additional findings from a recent floristics study of the Florissant site in Colorado. These charts allow easy comparison of the timing, floral composition, and taxon-richness at the genus level and higher for the 9 sites. Among the pre-Neogene sites discussed here are early and middle Eocene deposits in Canada and middle Eocene deposits in Idaho andWyoming that provide evidence of an early pulse in the post-Cretaceous radiation of freshwater diatoms. The survey of the fossil diatoms from lake sediments of the Florissant Formation in Colorado showed that this deposit contains the most diverse (genus-rich) early freshwater diatom flora on record. This late Eocene flora suggests a later pulse in freshwater diatom radiation during pre-Neogene time that contributed to the rich taxonomic diversity of today. The Florissant fossil diatom investigations yield a total of 33 freshwater diatom genera. This number exceeds by 20 genera any other single known non-marine diatom-bearing Eocene and older deposit. The Florissant diatom flora consists of taxa with decidedly modern affinities, has superior diversity (taxon-richness), and yields the first-reported occurrence in the geologic record of 14 freshwater genera. From the 9 featured pre-Neogene sites, genera of centric, araphid, biraphid, and monoraphid morphologic groups are represented. While the findings of these studies are not inconsistent with the previously predicted timing of the first appearance of the major morphological diatom groups, the actual known fossil record post-dates the presumed timing of such morphological differentiation. Although the first occurrences in these deposits must be treated as apparent first occurrences due to the innate incompleteness of the fossil record, such findings confirm that species of many of the modern non-marine diatom genera had evolved prior to the end of the Eocene. It is anticipated that this synthesis of the pre-Neogene non-marine diatom floras will provide necessary paleontological evidence for interpreting the true geologic ranges of several diatom lineages. This work could facilitate future application of diatoms in biostratigraphic studies of fossil lake beds in hydrologic and climate change studies and in hydrocarbon exploration. Perhaps such data will generate insights to diatom phylogenetics that may lead to a greater understanding of their evolutionary histories. Further research could potentially relate changes in diatom forms to global events that might include climate shifts.