Conventional transmission electron microscopy (TEM), freeze-etching, and scanning electron microscopy (SEM) were used to study changes in the external morphology of Azotobacter vinelandii ATCC 12837 during cyst formation. The various methods of cell preparation used for SEM drastically affected the morphology of vegetative and early precystic forms, while little change was observed in the external appearance of late precysts and cysts. SEM preparations from which cyst exine was removed before observation appeared morphologically similar to frozen-etched cells which revealed the central body.