Crabapple Island (located in Bełdany Lake, NE Poland) was the site of concurrent study on the reproduction and survival of bank voles (Myodes glareolus) and yellownecked mice (Apodemus flavicollis) in 1994–2002. We evaluated the importance of reproduction and survival for seasonal population dynamics as well as the formation of a summer peak in population numbers. Yellow-necked mice started breeding earlier in the spring than did bank voles. However, early breeding and rapid increase in numbers of both species resulted in a particular seasonal distribution of pregnant female numbers indicative for delayed (even up to 40 days) maximum reproduction of yellow-necked mice as compared with that of bank voles. A high survival rate of mature females of the yellow-necked mouse preceeds the July peak in population numbers, and a low survival rate of immature individuals of this species contributes to a rapid decline of numbers following this peak. The survival rate of mature females of the bank vole does not affect the summer peak in population numbers of this species. A relatively high survival rate of immature individuals of bank voles following the summer peak results in a slow decrease of its population size.