Previous work has shown that latent infections of Plasmodium relictum in English sparrows become patent in the spring of the year. This spring relapse phenomenon may be ecologically important in reestablishing transmission of the parasite if the increase in parasitemia is associated with an increased infectivity to mosquitoes. The present study compared laboratory transmission rates during winter and spring of chronically infected sparrows. Results of the study indicated that birds were significantly more infective to mosquitoes during the spring relapse period than during the winter latent period.