This contribution results from an investigation of four commercially obtained colonies of Bombus (Pyrobombus) impatiens Cresson in an attempt to understand the functional anatomy and behavior of its immature stages. Eggs are described in detail, and their chorionic microstructure is contrasted with that of Bombus (Cullumanobombus) griseocollis (De Geer). They are deposited in groups consisting of a few to more than nine eggs into a single chamber. The study confirms that larvae pass through four instars. Although increasing in size dramatically from one instar to the next, larval anatomy and behavior change little during the first three instars. The last instar is the one that commences defecation and production of silk resulting in cocoon construction. In contrast to most bees, its larval activities result in substantial changes in size and shape of its brood chamber, whereas brood chambers of most bees are constructed by the female and modified little by the offspring. This study is a part of a series of investigations into the similarities and differences in the developmental biologies among corbiculate bees.