The uncertain state of the traditional nonprofit has sparked interest in unincorporated cultural associations to maintain the vitality of the cultural sector. Despite increasing interest in and qualitative study of the role of the unincorporated groups and individuals in cultural production and participation, there are no data that allow assessment of their importance to the overall cultural sector. In this paper, SIAP takes an alternative strategy for estimating the informal arts sector. The authors use a representative sample of artists to ask what proportion of artists’ professional activities takes place in the for-profit, nonprofit, and informal sectors. The analysis is based on a sample of 270 artists in the Philadelphia metropolitan area interviewed during 2004. The team found that a large share of the sample’s professional activities did indeed occur in what might be called the informal cultural sector; and that the importance of this sector varied by discipline, age, and ethnicity of the artist. The informal arts sector is likely to be a major agenda item for cultural research in the years to come. If nothing else, this paper demonstrates that researchers can use quantitative methods to expand our understanding of the informal sector. It also holds out the promise that the research would contribute to a more complex and variegated portrait of informal cultural engagement and its place in the ecology of urban culture.