Abstract Reyna-lopez, G. and Ruiz-Herrera, J. 1993. Polyamines and the phorogenesis of mucorales. Experimental Mycology 17, 79-89. Appearance of sporangiophore primordia of Mucor rouxii and Phycomyces blakesleeanus was preceded by a transitory elevation in the levels of ornithine decarboxylase (ODC) and the polyamines putrescine and spermidine. If the ODC competitive inhibitor 1,4-diamino-2-butanone (DAB) was added before this time, it blocked the appearance of sporangiophores as well as the elevation of ODC and putrescine levels, but not those of spermidine, nor did it affect the growth of vegetative mycelium. If added later on, the drug did not prevent photogenesis, nor the differentiation of primordia into macro- or microphores. The effect of DAB on phorogenesis could be counteracted by putrescine (but not by spermidine) and by 5-azacytidine. It is suggested that polyamines are necessary to trigger the differentiation of some vegetative hyphae into sporangiophore primordia, probably affecting the expression of specific genes through a process involving DNA hypomethylation.