Asexual reproduction (conidiation) in Aspergillus nidulans is induced by environmental signals like exposure to air or nutrient starvation, and depends on brlA gene activation. The study of 'fluffy' mutants showing delayed asexual development and reduced brlA expression has defined the fluG pathway, involved in regulation of this differentiation process. Genetic characterization of a 'fluffy' mutant identified tmpA as a new gene involved in regulation of conidiation. TmpA defines a new family of putative transmembrane proteins of unknown function, widespread in filamentous fungi and plants, with homologues showing similarity to non-ribosomal peptide synthetases. The deletion of tmpA resulted in decreased brlA expression and conidiation in air-exposed colonies. This defect was suppressed when DeltatmpA mutants were grown next to wild-type or DeltafluG mutant colonies, even without direct contact between hyphae. In liquid culture, tmpA was essential for conidiation induced by nitrogen but not by carbon starvation, whereas the overexpression of different tmpA tagged alleles resulted in conidiation. The overexpression of fluG-induced conidiation independently of tmpA and DeltatmpADeltafluG double mutants showed an additive 'fluffy' phenotype, indicating that tmpA and fluG regulate asexual sporulation through different pathways. TmpA and its homologues appear to have diverged from the ferric reductase family, retaining overall transmembrane architecture, NAD(P), flavin adenine dinucleotide (FAD) and possibly haem-binding domains. Based on our results, we propose that TmpA is a membrane oxidoreductase involved in the synthesis of a developmental signal.