The behaviour and the morphology of the infection structures of the monokaryotic phase ofCronartium flaccidumestudied on needles ofPinus pineaseedlings inoculated with the rust basidiospore. It was found that the penetration and early colonization structures ofC. flaccidumin the monokaryotic phase maintained the morphological and functional significance of the typical monokaryotic ones, even if some aspects of their behaviour seemed to recall those of the dikaryon. A possible hypothesis as to the reason for the dynamics of penetration carried out on the markedly cutinized needles of pine byC. flaccidumin the monokaryotic phase is discussed. It can be concluded that inC. flaccidumin the monokaryotic phase it is the nuclear set which determines the morphology and function of the structures involved in the infection process; this is true even if the histological characteristics of the host organ which this rust species has evolved to infect in nature, condition its way of penetration.