The life histories of Apodemus sylvaticus, Clethrionomys glareolus caesarius, Microtus arvalis sarnius and Crocidura russula have been described for 1960-61 from animals collected during the course of five visits to the Channel Islands of Jersey, Guernsey, Alderney and Sark between March 1960 and February 1961. In Apodemus the overwintering population started breeding in March 1960 and continued to do so until it disappeared from the population in late autumn. In the latter part of the summer much breeding was performed by animals born earlier that year. The pattern is similar in Clethrionomys except that breeding started rather later. In Microtus pregnant females were obtained between March and October. In Crocidura russula animals born in 1960 did not breed that year. The breeding season lasted from late February until October when the last of the animals born in 1959 were caught.