In A. fumigatus, 13 HKs have previously been identified, and three of them have been investigated. A deletion mutant of the fos1 gene showed a moderate resistance to fungicides and attenuated virulence. The tcsB deletion mutant showed a slight sensitivity to sodium dodecyl sulfate and growth inhibition under high temperature conditions. TcsC, a group III histidine kinase, was recently characterized by McCormick et al. as described below. Characterization of the other HKs would be helpful to improve the understanding of the TCS signaling circuitry in A. fumigatus. Among the different types of HKs, a large amount of attention has been paid to group III HKs. This gene was initially identified in N. crassa as an osmotic stress-sensitive mutant allele, and later it was identified as a dicarboximideresistant mutant allele. This HK possesses a characteristic motif in its N-terminal region, consisting of four to six repeats of the HAMP domain. Although the functions of the motif were obscure, a null Mepiroxol mutation and deletion of the gene resulted in resistance to the dicarboximide and phenylpyrrole fungicides in all fungi that possess this type of HK in its genome. Intriguingly, although S. cerevisiae has no Nik1-type HK, heterologous expression of Nik1-type HKs from other species made S. cerevisiae responsive to these fungicides. These findings illustrated that Nik1-type HKs play a crucial role in the fungicide action and that the mode of action is convertible across some fungi. Furthermore, a recent striking finding for this type of HK is its involvement in dimorphic switching in dimorphic pathogens including Penicillium marneffei, Histoplasma capsulatum, and Blastomyces dermatitidis. However, the detailed molecular mechanism has yet to be elucidated. Recently, the characterization of TcsC, a Nik1-type HK of A. fumigatus, has been reported by McCormick et al.. TcsC is required for hyper-osmotic stress adaptation and sensitivity to certain fungicides such as fludioxonil, as well as phosphorylation of the SakA MAPK in response to these stimuli. Despite these phenotypes in vitro, TcsC seemed to be dispensable for virulence. In this study, we characterized the Nik1-type HK of A. fumigatus with regard not only to its role in osmotic stress and fungicide responses, development, and morphology, but also with regard to its role in the signaling pathway associated with the SskA response regulator and SakA MAPK, which are downstream components of the HOG pathway. Furthermore, the involvement of the other HKs in the HOG pathway was investigated. In this study, we found that the absence of NikA resulted in pleiotropic phenotypes such as growth retardation, reduction of conidia, aberrant hyphae, 4-(Benzyloxy)phenol tolerance to cell wall-perturbing reagents and fungicides, and marked sensitivity to high osmolarity stress. However, NikA is likely to function independently of SskA and SakA in these phenotypes except for osmotic adaptation, which raised the question of what alternative component functions downstream of NikA. Because NikA is a component of the His-Asp phosphorelay circuitry in the TCS, NikA is thought to regulate RRs via the YpdA HPt in a phosphorelay-dependent manner. Given that SskA is not involved in the phenotypes, a plausible candidate is AfSkn7, the other type of RR in A. fumigatus. InA. nidulans, the double gene deletion of SskA and SrrA, which is an ortholog of AfSkn7, resulted in a DnikA-level tolerance to the fungicides and a marked sensitivity to osmotic stress. A similar result was also reported in Cochliobolus heterostrophus, suggesting that SskA and SrrA/Skn7 are redundantly or cooperatively involved in the adaptations to fungicides and osmotic environment downstream of NikA.