Rotaviral infections in cynomolgus monkeys (Macaca fasicularis) were studied to ascertain its suitability as a model of infection and diarrhea caused by group A human rotaviruses. Formula-fed monkeys were used as they could be observed closely. Experimental rotaviral infection of cynomolgus monkeys was age-dependent; only young monkeys were readily infected. Formula-fed newborns were readily infected with cell-culture-adapted human (WA) and simian (SA11) viruses and with a rotavirus from a human fecal specimen. However, diarrhea was detected only in very young animals. A number of rotaviral shedding patterns as a function of time were observed. Although there was no typical viral shedding pattern which represented exclusive association of viral infection with diarrhea, the initial level of viral excretion and the maximum level of viral shedding attained were much higher in animals with diarrhea. Seroconversion occurred in less than half of the inoculated animals. The presence of maternal rotaviral antibodies did not prevent infection or diarrhea.