Noroviruses (NoV) are a leading cause of outbreaks of nonbacterial acute gastroenteritis in humans worldwide and have become an important cause of hospitalization of children in South America. NoV belong to the family Caliciviridae and are non-enveloped single stranded, positive sense, RNA viruses. NoV of genotype GII/4 have emerged worldwide, causing four epidemic seasons of viral gastroenteritis during which four novel variants emerged. Despite the importance of NoV outbreaks, little is known about the evolutionary rates, viral spread and population dynamics of NoV populations. In order to gain insight into these matters, a Bayesian Markov chain Monte Carlo (MCMC) approach was used to analyze region D or full-length VP1 gene sequences of GII/4 NoV populations isolated in Brazil or Japan, respectively. The results of these studies revealed that the expansion population growth model was the best to fit the data in both datasets. The dates of the most common recent ancestors revealed that these viruses can quickly emerge in a geographical location. A mean evolutionary rate of 1.21 x 10(-2) nucleotide substitution/site/year (s/s/y) was obtained for the VP1 gene using full-length sequences. This rate is higher than the rates reported for other rapidly evolving RNA. Roughly similar rates (1.44 x 10(-2)s/s/y) were found using region D sequences, revealing the suitability of this region for evolutionary studies, in agreement with previous reports. High evolutionary rates and fast population growth may have contributed to the vigorous initial transmission dynamics of the GII/4 NoV populations studied.