Entomopathogenic fungi, such as Isaria fumosorosea, Beauveria bassiana, Metarhizium anisopliae and Lecanicillium spp., are all commercially available as microbial pesticides (Copping, 2009). However, entomopathogenic fungi can be negatively affected by fungicides, with the result that their biological control potential can be reduced if both products are used together (Clark et al., 1982; Loria et al., 1983; Saito & Yabuta, 1996; Pell et al., 2010; D'Alessandro et al., 2011). While the harmful effects of fungicides on entomopathogenic fungi are typically avoided by employing extended intervals between the application of each agent (Gardner et al., 1984; Bruck, 2009), these intervals can complicate the effective and practical use of both products in the field. One way to overcome this problem has been to develop fungicide-resistant isolates of entomopathogenic fungi by selection on chemically amended media (Shapiro-Ilan et al., 2002, 2011; Butters et al., 2003), by transformation (Pfeifer & Khachatourians, 1992; Inglis et al., 1999), or by exposure to mutagenic agents such as NaNO2 (Zou et al., 2006; Song et al., 2011). Irradiation with ion beams is also a useful method for mutagenesis in microorganisms (Matuo et al., 2006; Tanaka et al., 2010; Toyoshima et al., 2012). For example, exposure of the entomopathogenic fungus Cordyceps militaris to ion beams successfully generated a mutant isolate capable of enhanced production of cordycepin, a medicinal adenosine analogue (Das et al., 2008, 2010). However, there is relatively little information in the literature on the use of ion-beam irradiation to induce mutations in entomopathogenic fungi and none considering induction of mutations conferring resistance to fungicides. Gamma-ray irradiation has also been demonstrated as a successful mutagenic agent in entomopathogenic fungi (Paccola-Meirelles & Azevedo, 1991; Kava-Cordeiro et al., 1995), although there are no reports describing enhanced fungicide resistance induced by gamma-ray irradiation. In this study, an isolate of I. fumosorosea was subjected to ion-beam and/or gamma-ray irradiation to encourage the production of mutants with the potential for increased resistance to benomyl as a model fungicide. Levels of benomyl resistance in the mutants and their responses to other fungicides at recommended field application rates were also examined. Moreover, the β-tubulin locus of the mutants was sequenced, as benomyl resistance in B. bassiana is known to be conferred by a mutation at this locus (Butters et al., 2003; Zou et al., 2006).