Deletion of the spermidine synthase gene in the fungus Aspergillus nidulans results in a strain, deltaspdA, which requires spermidine for growth and accumulates putrescine as the sole polyamine. Vegetative growth but not sporulation or sterigmatocystin production is observed when deltaspdA is grown on media supplemented with 0.05-0.10 mM exogenous spermidine. Supplementation of deltaspdA with >/= 0.10 mM spermidine restores sterigmatocystin production and >/= 0.50 mM spermidine produces a phenotype with denser asexual spore production and decreased radial hyphal growth compared with the wild type. DeltaspdA spores germinate in unsupplemented media but germ tube growth ceases after 8 h upon which time the spores swell to approximately three times their normal diameter. Hyphal growth is resumed upon addition of 1.0 mM spermidine. Suppression of a G protein signalling pathway could not force asexual sporulation and sterigmatocystin production in deltaspdA strains grown in media lacking spermidine but could force both processes in deltaspdA strains supplemented with 0.05 mM spermidine. These results show that increasing levels of spermidine are required for the transitions from (i) germ tube to hyphal growth and (ii) hyphal growth to tissue differentiation and secondary metabolism. Suppression of G protein signalling can over-ride the spermidine requirement for the latter but not the former transition.