## 1 Introduction  
Dimorphic fungi have a unique attribute of transitioning between yeast and mycelial phases. Environmental stimuli such as temperature, pH, and nutrition influence this morphological change [1; 2; 3; 4]. Several fungal pathogens of animals and plants are dimorphic, and the transition between the two phases favors fungal in vivo survival and pathogenesis [5]. Breakthroughs at the molecular level reveal the mechanisms underlaying the infection process in which virulence factors aid in pathogen colonization, either during the transition of mycelia to the parasitic phase or during host immunosuppression [6; 7; 8]. For example, molecules such as branched-chain amino acids synchronize biological events related to the dimorphic transition in _Certocytosis_ (_Optiotsoma_) _ulmi_, the causal agent of Dutch Elm disease [1; 9]. Likewise, hydroxy fatty acids stimulate filament formation in _Uslitago maydis_[10]. Moreover, certain volatile organic compounds (VOCs) such as alcohols have been reported to regulate fungal cell elongation, nuclear division, budding pattern, and condiation [11; 12; 13]. For example, 3-methylbutanol promotes the continuous budding phenotype in _Saccharomyces cerevisiae_, and the aromatic alcohol tyrosol and sesquiterpene farnesol control physiological activities in the polymorphic opportunistic fungus _Candidia albicans_[5; 11; 13; 14; 15; 11; 16].  
_Beauveria bassiana_ is a dimorphic fungus that infects arthropods of all major insect orders [17; 18]. Furthermore, _B. bassiana_ survives in the soil and can colonize different partsof the plant as endophytes [19; 20]. Virulence among _B. bassiana_ strains varies, however, and the physiological state of the insect host is an important factor for successful infection [21; 22]. Usually, the fungal spores adhere to and germinate on the insect surface and penetrate the insect cuticle until they reach the hemocoel; here, the mycelium transforms to yeast-like cells called blastospores or hyphal bodies. During the last stage of the infection, blastospores transit to mycelia and emerge from the insect cadaver, producing newly infective conidia that are disseminated towards a new insect host, thus starting a new cycle of infection [18]. Recently, crucial activator genes such as _BbbrIA_ and _Bbaba_ of the central developmental pathway have been described as regulators of the fungal transition from filamentous form to blastospore and aerial condiation [23]. However, the involvement of fungal quorum sensing-like molecules modulating dimorphic switching in _B. bassiana_ remains unknown. Nevertheless, the mycotoxins produced, such as beauvericin, basianolide, and oosporein, act simultaneously as diffusible chemical signals for fungal development, aggressiveness and/or virulence [8; 18; 22].  
Indirect interactions between _B. bassiana_ and its insect host can also occur via VOC emissions. Thus, prior to colonization, fungal VOCs modulate different types of behaviors, such as oviposition [8; 24; 25; 26; 27; 28].  
This study was based on the fact that the yeast-to-mycelial conversion in dimorphic fungi is affected by external factors and is important for pathogenesis and virulence. Here, we studied two strains of _B. bassiana_ exhibiting varied virulence. One of the strains showed an early dimorphism, which was associated with an increased toxicity of culture filtrate. Additionally, we analyzed volatile molecules that regulate fungal morphogenesis via quorum sensing (QS). Our findings provide new information on the emission of volatiles during the fungal life cycle and add to our knowledge on the in vitro growth of _B. bassiana._