Aging of thalli of two microsclerotial strains ofVerticillium albo-atrumisolated from tomato and avocado in Morocco affected microconidia viability and the phenotype of regenerated thalli. The percentage of regeneration of microconidia harvested in thalli of increasing age was first high; it diminished to a minimum, then increased again reaching high values. The age of cultures for which these variations arose as well as the different values of percentage of regeneration were dependent on the isolate and on culture conditions. Microconidia from young cultures produced only wild thalli; microconidia from old cultures also regenerated stable morphological variants, which increased in number with the age of the culture. Variants were numerous only when the age of cultures was equal or higher than the age for which the percentage of regeneration was minimum; they probably arose from late forming microconidia on old thalli.