Variations in the quantity of secretory organelles (trichocysts) within clonal populations is investigated, with particular emphasis on the effects of long-term maintenance with or without 63 nM selenium (Se) in the growth medium. The principal results concern clones that produce many trichocysts when continuously grown with Se, and in which no cells are observed without trichocysts. When grown without Se, no cells produce trichocysts. When clones are grown for various durations of up to 4 years without Se, and again placed in growth medium with Se to allow trichocyst production, they show clonal segregation in the quantity of trichocysts produced: cells without trichocysts, or with few trichocysts, are observed among the cells with many trichocysts. Subcloning reveals that some of the cells without trichocysts are pure non-secreters, since they do not regain ejectable trichocysts even following growth with Se for 5 years, whereas cells with many trichocysts continue to be strong secreters if maintained with Se. To explain these observations, and to provide a working hypothesis for future study, a secretion factor (SF) is proposed to be implicated in processing of trichocyst precursors to final secretory products. Se is postulated to be necessary for SF gene expression and/or replication, which become irreversibly blocked during long-term Se deficiency.