In the genome of Aspergillus nidulans, a defensin-like protein, Anisin1, was annotated with high homology to the mosquito defensin AaDefA1. So far, no studies exist on defensins from filamentous ascomycetes. Therefore, we characterized the anisin1 gene in A. nidulans and generated a deletion mutant, which suffered from a defect in mitospore development and produced less conidia at 42°C compared to the reference strain. In surface cultures of A. nidulans wild type, the anisin1 expression correlated with that of the central regulator for asexual development, brlA, and with the major scavanger of H(2)O(2) stress, catB, which is indicative for cell differentiation in developing fungi. Interestingly, brlA and anisin1 expressions were deregulated in a ΔsrrA strain that covers a central role in the histidine-to-aspartate (His-Asp) phosphorelay signaling pathway and shows impaired asexual development and H(2)O(2) detoxification. In submers cultures of A. nidulans wild type and other mutants of the His-Asp phosphorelay signaling pathway, anisin1 was repressed, but derepressed in a ΔsrrA background, and anisin1 transcription was further increased in this mutant by H(2)O(2) addition. We therefore conclude that the secreted protein Anisin1 contributes to the optimal development of A. nidulans and we further propose that it has a sensing/signaling function for elevated H(2)O(2) levels.