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Bark beetles are associated with diverse fungal symbionts. One of these fungi, with poorly known biology belong to the genus <i>Geosmithia</i> (Ascomycota: Hypocreales). The current knowledge is based on samples from western Palearctic, USA and several other regions. 50 species of the genus (22 published, 11 formally described) were found until now. <i>Geosmithia</i> spp. are typically associated with phloeophagous bark beetles, bostrichids and weevils, and over of 90% from the screened insects species were found to be obligate or facultative vectors. A derived ecological strategy, found in three <i>Geosmithia</i> spp., represents association with ambrosia beetles, where they act as primary ambrosia fungi. Several species occur on alternative substrates or can live as endophytes of healthy trees. One species, <i>G. morbida</i>, was proven to be a pathogenic fungus causing tissue necroses on walnuts. <i>Geosmithia</i> community pattern and presence of primary ambrosia fungi suggest the long-term and stable association between <i>Geosmithia</i> and bark beetles. <i>G. morbida</i> belongs to clade of hardwood-associated species. Major enzymatic activities, utilization of C, N, P, S sources (BIOLOG microarrays), secondary metabolites and antimicrobial activities of <i>G. morbida</i> are presented. Physiological fingerprinting places <i>G. morbida</i> among strict specialists confirming its specificity to a single host and long term co-evolution.<p><p>Keywords: