The living parasites in Toxoplasma cysts cannot be eradicated by current therapy and maintain latent infections for many years. Relatively little is known about encysted Toxoplasma. We therefore undertook studies using mice infected with the avirulent ME 49 strain of Toxoplasma. The bradyzoites in young (12- to 17-day-old) cysts contained the same organelles as did tachyzoites. The bradyzoites of older cysts (4 weeks postinoculation) had differentiated, losing certain organelles and acquiring others. Our major new finding was that in animals inoculated greater than or equal to 4 weeks previously, some bradyzoites were totally disrupted, spilling their contents (perhaps including lytic substances) into the cyst matrix. Many older bradyzoites in the same cysts lacked internal membranes and their viability was questionable, but there were also occasional parasites resembling viable tachyzoites and mature bradyzoites, organisms that might possibly initiate daughter cyst formation after cyst rupture. The life span of an individual bradyzoite may be shorter than formerly appreciated despite the prolonged course of latent infections.