Whenchloramphenicol was addedtoa culture ofBacillus subtilis inearlyexponential growth, microscopic observation ofcells stained by4',6-diamidino-2-phenylindole showednucleoids thathadchanged inappearance fromirregular spheres anddumbbells tolarge, brightly stained spheres andovals. Incontrast, theaddition of chloramphenicol tocultures inmid-andlate exponential growth showedcells withelongated nucleoids whose frequency andlength increased astheculture approached stationary phase. Thekinetics ofnucleoid elongation after theaddition ofchloramphenicol toexponential-phase cultures was complex. Immediately after treatment, therateofnucleoid elongation was veryrapid. Thenucleoid thenelongated steadily forabout4min,after whichtherateofelongation decreased considerably. Nucleoids ofcells treated with6-(p-hydroxyphenylazo)- uracil (aninhibitor ofDNA synthesis) exhibited theimmediate rapidelongation upon chloramphenicol treatment butnotthesubsequent changes. Theseobservations suggest thataxial filament formation during stationary phase(stage Iofsporulation) intheabsence ofchloramphenicol results fromchanges innucleoid structure that areinitiated earlier, during exponential growth. Manyinvestigators havesought todescribe thearrange- mentofthenucleoplasm intheprocaryotic cell. Thesubject hasremained elusive, partly because thenucleoid undergoes rearrangement during themostgentle manipulations (e.g., rapidfiltration andtemperature changes, etc.(8)). Thus, after decades ofeffort, onlya fewnoncontroversial state- mentscan bemade:thenucleoid oftheexponential-phase cell islocalized primarily inthecenterofthecellbutdoes extend totheperiphery ofthecytoplasm (2,7,8,12,13); in stationary-phase cells, thenucleoid becomesconfined tothe center, andthisresults ina decrease inthedensity ofthis portion ofthecell(3,6,21). Recently there hasbeena resurgenceofinterest innucle- oidstructure. Inpart, this hasresulted fromthedevelopment ofcryofixation techniques forelectron microscopy (13), the availability ofDNA-specific fluorescent stains suchas4',6- diamidino-2-phenylindole