Pheromone-initiated sexual reproduction of Volvox carteri f. nagariensis is started by a “spontaneous” generation of sexual male spheroids at a rate of 10-4. As previously suggested, these males are mutants (sex_c) which become independent of pheromone-controlled sex induction. Unequivocal evidence for a mutative event now comes from the observation that the sex0 trait is transmitted through cell lineages during embryogenesis, thus confirming earlier morphological analysis. Mutation seems to be restricted to cell division. The high frequency of mutation points to a special enhancing mechanism for which the possibility of a genome rearrangement is discussed.