The cell cycle and the relationship between particular cell cycle phases and the differentiation of trophozoites into cysts were reinvestigated in Acanthamoeba castellanii using flow fluorometric measurements of nuclear DNA content and synthesis and synchronization of cells by release from the stationary phase. The investigation was performed with cultures growing in non-defined medium (ND cells) showing a high degree of encystation in response to starvation and with subcultures growing in chemically defined nutrient medium (D cells) exhibiting a very low encystation competence. In both cultures the cell cycle starts with a short S phase taking place simultaneously with cytokinesis followed by a long G 2 phase. A G 1 phase seems to be either absent or very short. Synchronization experiments reveal that in ND cells encystation is initiated from a particular position of late G 2 . The high encystation competence of stationary phase ND cells seems to be due to arrest of cells at this particular cell cycle position. The lack of encystation competence of stationary phase D cells correlates with the loss of accumulation of cells at this particular stage of the cell cycle. This change of the property of cells is related to the growth condition and not to an irreversible loss of encystation competence of D cells.