Previous studies (Aufauvre-Brown et al., 1997; Mellado et al., 1996a,b ) have shown that only two genes of the Aspergillus fumigatus chitin synthase family, chsG and chsE, play a role in the morphogenesis of this fungal species. An A. fumigatus strain lacking both chsG (class III CHS) and chsE (class V CHS) genes was constructed by gene replacement of the chsE gene with a copy that has its conserved coding region interrupted by the hph resistance cassette in an A. fumigatus chsG- genetic background. Unexpectedly the double disruption was not lethal. The double mutant AfchsG-/chsE- strain (i) has reduced chitin synthase activity with or without trypsin stimulation, (ii) has a reduced colony radial growth rate, (iii) produces highly branched hyphae, (iv) exhibits aberrant features, such as periodic swellings along the length of the hyphae and a block in conidiation that can be partially restored by an osmotic stabilizer (v) shows alterations in the shape and germination capacity of the conidia, and (vi) has a cell wall that contains half the chitin of the parental strain and is, unexpectedly, highly enriched in alpha-(1-3) glucan.