We report a case of cystitis due to Toxoplasma gondii in a patient with AIDS who presented with dysuria and urinary frequency. To our knowledge, this is the first reported case of cystitis due to this organism. Microscopy of bladder specimens revealed inflammatory cystitis, with Toxoplasma cysts disseminated within the mucosa. No other pathogen could be detected by urine culture, cytoscopy, or staining of bladder specimens obtained at autopsy. Diagnosis of cystitis due to Toxoplasma gondii may be difficult because this illness is associated with misleading radiologic and endoscopic findings. Toxoplasmosis is a rare but potentially curable cause of culture-negative cystitis in patients with AIDS.