Around the age of two, many children enter a period of rapid vocabulary growth. The Naming Boom is commonly conceptualized as an increase in the rate of word learning across all categorical domains. However, recent research has suggested that the development of rapid word learning may occur within specific categorical domains as the result of previous experience learning words within the domain. The current study further tested this idea by analyzing patterns in childrens vocabularies at the onset of the Naming Boom. Childrens vocabularies were assessed; words that each child knew were sorted according to categorical domains (such as animals or vehicles). Most vocabularies exhibited a pattern of domain clumping: children knew many words in some domains, but few words in others. Results lend further support the to idea that rapid vocabulary growth develops within specific categorical domains and suggests that the Naming Boom may be better conceptualized as a series of domain-specific boomlets.