Rcccivcd May 2, 1983WILLIAMS, L. M., and J.-G. LAFONTAINE. 1984. Ultrastructural changes in thc myxamoebac of Physrri.~lii~ po!\;ce~~/~ol~cm in response to a microcyst-inducing concentration of mannitol. Can. J. Bot. 62: 272-280. The response of axcnically cultured PIzysclrlrin polycephol~tin myxarnocbae to a microcyst-inducing conccntration of mannitol (0.5 M ) has bccn studied for both log-phasc and maximum-concentration cultures. Results inclicatc that mannitol alone is not sufficient to induce cncystmcnt; a population effect is also necessary. Myxamocbac may continue to dividc in the presence of mannitol if this effcct is abscnt. Early ultrastructural changcs have bccn notcd indicating that thc primary mode of action of mannitol is via the change in osmotic potential of thc medium. Nuclcar and cytoplasmic ultrastructural changcs during the cncystmcnt proccss are documcntcd. Rccovcry of log-phase cclls to undergo mitosis involvcs dcfinitc morphological changes, which are also dcscribcd. Ruthenium rccl staining was utiliscd to cmphasizc changcs in the ccll coat and indicate possiblc sitcs of accumulation of ccll wall material. WILLIAMS, L. M., et J.-G. LAFONTAINE. 1984. Ultrastructural changcs in thc myxamocbac of Phystrr-LOTI polycepl~nl~rm in responsc to a microcyst-inducing conccntration of mannitol. Can. J. Bot. 62: 272-280. Nos observations portent sur I'cnkystemcnt d'amibcs dc Phy.stri.um po!\'cept~trl~rii~ cn prCsence dc mannitol 0.5 M.Les rCsultats obtenus indiquent quc le ~nannitol scul nc suffit pas pour induire l'cnkystcmcnt mais quc la conccntration dcs amibes est aussi impliquke. Si ccttc conccntration dcs amibes n'cst pas asscz ClevCc ccs dcrnikrcs continuent i~ sc diviscr en presence dc mannitol. Lcs changcmcnts prCcoccs dc I'infrastructurc dcs amibcs indiqucnt quc I'action du ~nannitol a licu via la modification du potcnticl osmotiquc du milieu. Ccrtains dcs changcmcnts infrastructuraux du noyau et du cytoplasmc au cours du phCnomknc d'enkystcmcnt sont dCcrits. On dCcrit aussi ccrtaincs modifications morphologiques qui ont licu lorsque dcs ccllules en phase logarithmique entrcnt cn division. L'utilisation du rouge de ruthCnium comme colorant permet de mieux visualiser I'Cvolution dc I'cnveloppe cellulairc.