Epidemic influenza is typically caused by infection of viruses of the A and B type and can result in substantial morbidity and mortality during a given season. Here we demonstrate that influenza B viruses can replicate in the upper respiratory tract of the guinea pig, and that viruses of the two main lineages can transmit with 100% efficiency between inoculated and na?ve animals in both contact and non-contact models. Our results also indicate that, like influenza A virus, transmission of influenza B viruses is enhanced at colder temperatures, providing an explanation for the seasonality of influenza epidemics in temperate climates. We therefore present, for the first time, a small animal model in which to study the underlying mechanisms of influenza B virus transmission