The filamentous fungus Aspergillus fumigatus is the major cause of life threatening invasive aspergillosis, and its small hydrophobic asexual spores (conidia) are the major infection agent. To better understand biology of A. fumigatus, we have characterized the rax1 gene encoding a putative regulator of G protein signaling (RGS). The deletion (Delta) of rax1 results in restricted colony growth and highly reduced number of conidia in A. fumigatus. Transcript levels of the three central activators of asexual development abaA, brlA, and wetA are significantly reduced in the Delta rax1 mutant. However, the Delta rax1 conidia, but not vegetative cells, are specifically resistant against H2O2 stress. The Delta rax1 conidia accumulate higher mRNA levels of sakA encoding a key MAP kinase for stress response. Moreover, the Delta rax1 conidia contain over five-fold amount of trehalose, an osmolyte and protein/membrane protectant. Transmission electron microscopy analyses indicate that the Delta rax1 conidia have the thicker melanized-outermost cell wall layer compared to those of wild-type. In summary, Rax1 positively controls growth and development, and modulates intracellular trehalose amount, cell wall melanin levels in conidia, and spore resistance to H2O2. (C) 2017 Elsevier Inc. All rights reserved.