Although the Chestnut-sided Warbler (Dendroica pensylvanica) is one of the most familiar parulids to most modern observers, the development of the nestling, and the nesting success of the species in Michigan have apparently been little studied. During July of 1962 and from early June to mid-August of 1963 and 1964, I studied 11 nests of the Chestnut-sided Warbler in the vicinity of UMBS, Cheboygan County, Michigan. Observations on two nests began while nest-building was in progress, on seven during egg-laying or early stages of incubation, and on two after hatching of the eggs. Detailed observations were made from a canvas blind placed on the ground within 5 feet of the nest. Pre- and post-nesting activity, and nest building were observed without the use of blinds.