The deposition of amyloid beta (A beta) protein in the brain has been demonstrated immunocytochemically in the small Lemurian primate Microcebus murinus. Both meningocerebral vascular deposits and cortical parenchymal deposits occur. All eight aged (> 8 years old) Microcebus examined showed vascular amyloid deposits, whereas only four exhibited parenchymal plaques. The vascular amyloid infiltrated the tunica media of the leptomeningeal and cortical arteries and arterioles and was also found in capillaries. A beta was observed to be deposited in three general forms in the cortical neuropil: round or elliptical plaques that were thioflavin-negative but sometimes showed a central concentration of A beta immunoreactivity; round plaques with a densely immunoreactive core that was thioflavin-positive; extensive ribbon-like infiltrations enclosing multiple cortical blood vessels. These observations, taken together with previous descriptions of age-related neurodegenerative changes in Microcebus, indicate that this species undergoes a beta-amyloid-associated neuropathology highly similar to that seen in Alzheimer's disease. We conclude that this lemurian primate of small size and relatively short life expectancy, provides a compelling animal model of some principal features of Alzheimer's disease.