The fidelity, within three breeding seasons, of 109 catbirds, Dumetella carolinensis, to their mates and breeding territories was studied. About one-third of the birds abandoned their territories, usually after a nesting failure. Most deserted as pairs and established new territories up to 450 m distant from the original. The causes of desertion could not usually be determined: neither the number of nest losses preceding desertion nor the period in the nesting cycle when desertion occurred was clearly influential. Excluding those pairs in which one member of a pair died, only one bird was known to change mates after a nesting failure. The pair-bond remained intact in 15 of 16 pairs that attempted second broods. The reproductive success of pairs faithful to their territory was greater, but not significantly so (P =.143), than that of pairs that deserted.Patterns of desertion in passerines and the advantages of desertion are discussed.